LONDON, England (CNN) -- Security across London has been increased after police on Friday discovered two cars filled with explosives.

Meanwhile, police increased patrols across London in a hunt for what they said is a man seen running from one of the cars on Friday.

British officials said hundreds of people could have been killed if the devices in the cars had been set off.

The first car was discovered parked near Piccadilly Circus; the second was found about an hour later, less than a kilometer away near Trafalgar Square.

Scotland Yard authorities said they believed the two incidents were connected.

On Friday, London police said the second discovered car -- containing fuel, gas canisters and nails -- was "clearly linked" to the first explosives-packed car found outside a nightclub near Piccadilly Circus, Metropolitan Police said.

A "considerable" amount of fuel and gas canisters, along with a "substantial quantity of nails," was found in the blue Mercedes 280E, said Peter Clarke, Metropolitan Police deputy assistant commissioner, said about the second car.

He called the discovery of the second bomb "troubling," but urged the public to remain vigilant and report suspicious behavior to authorities.

The second vehicle was ticketed about 2:30 a.m. Friday (9:30 p.m. Thursday ET), Clarke said. It was near Trafalgar Square, roughly a half-mile from where the first vehicle -- also a Mercedes -- had been found about an hour earlier. (Watch how the second car was armed )

About 3:30 a.m., Clarke said, the Mercedes was taken to an impound lot in Hyde Park. Security sources earlier told CNN that workers who towed it thought the car smelled of gasoline, and became suspicious because of the reports that gasoline was among the explosive materials found in the first vehicle.

Clarke said the second device, like the first, was "potentially viable" but was rendered safe by police explosives officers.

"These vehicles are clearly linked," he said.

The first car, a silver Mercedes-Benz sedan, was discovered about 1:30 a.m. when an ambulance crew called to treat an ill person noticed what appeared to be smoke inside the car and notified authorities, London police said.

The car was parked in front of the Tiger Tiger club, and the discovery prompted the closing of several streets until the vehicle was hauled off nine hours later.

"In the car, they found significant quantities of petrol together with a number of gas cylinders," Clarke said. He could not immediately say how much fuel was there.

"I can tell you it was in several large containers," Clarke said. "There were also a large number of nails in the vehicle."

He said explosives officers manually disabled "a potential means of detonation for the gas and the fuel in the vehicle," which preserved crucial forensic evidence for investigators.

A cell phone was found as part of the device in the silver car, according to security sources with knowledge of the investigation, although it was not immediately known what role the cell phone may have played in the device. The sources said the device was apparently set up to be remotely detonated.

Metropolitan Police Counterterrorism Command officers are reviewing closed-circuit security camera video to see if they can determine who parked the car there, Clarke said.

London has a long history of bomb attacks and alerts during decades of violence mounted by Northern Irish guerrilla groups. Lone attackers also have previously targeted the city's gay and immigrant communities. (Timeline of attacks)

Friday's incident came days ahead of the second anniversary of July 7, 2005, when four Islamic extremist suicide bombers killed 52 people on London's transport system in the deadliest strike on the city since World War II.

Witness Daniel Weir said he was walking home from work when he noticed police had cordoned off the area around the nightclub and a nearby vehicle.

He snapped several photos, including one that showed a canister labeled "patio gas." (See the photos)

Clarke said it was too early to determine if the smoke the ambulance crew saw was an indication that the car bomb had been activated but failed to explode.

While Clarke would not speculate that Tiger Tiger was the target, he said "some features of what's happened resonate with previous plots."

"In one previous case we heard talk about nightclubs potentially becoming targets. ... We, of course, saw reference to vehicles being filled with gas or fuel in order to create an explosion," he said.

There had been no intelligence warning of an attack, he said.

"It is obvious that if the device had detonated there could have been significant injury or loss of life," Clarke said. "The vehicle was parked in one of the busiest parts of central London in the early hours of Friday morning when many, many people were leaving nightclubs and other places after the evening hours." (Watch police describe potential blast )

British Home Secretary Jacqui Smith said, "We're currently facing the most serious and sustained threat to our security from international terrorism."

The bombs were found just two days after new Prime Minister Gordon Brown took office, and one day after he appointed members of his Cabinet. (Full story)

"For Gordon Brown, it is a rude awakening to the realities you take on as prime minister," CNN's European political editor Robin Oakley said. (Watch Oakley comment on 'rude awakening' for Brown )

Brown, whose predecessor, Tony Blair, stoked anger among Islamic militants with his support for the Iraq war, said Britain faces "a serious and continuous threat" and the public "need to be alert" at all times.

The incident also came days ahead of the second anniversary of July 7, 2005, when four Islamic extremist suicide bombers killed 52 people on London's transport system in the deadliest strike on the city since World War II.

Atlantis and its seven astronauts returned to Earth safely Thursday, ending a two-week mission to deliver an addition to the international space station and bring home a crew member from the outpost.

Atlantis had to use its backup landing strip in California after rain and clouds over Florida ruled out Kennedy Space Center.

"We couldn't quite get comfortable with [Kennedy]," Mission Control told the Atlantis crew. "We are going to target Edwards."

The crew executed the landing on its first opportunity at Edwards at 3:49 p.m. ET.

Rain and clouds that scrubbed a planned Thursday landing in Florida continued Friday, and NASA waved off the first planned landing there, which would have been at 2:18 p.m. ET.

The landing site at Edwards had clear skies, and wind gusts there were not expected to pick up until late in the day.

"Our mind-set down here is we're going to land you somewhere safely today," Mission Control told the shuttle crew Friday morning.

Aboard Atlantis, the astronauts closed the shuttle's payload bay doors in preparation for landing. The doors are opened during flights so heat doesn't build up in the orbiter.

Atlantis commander Rick Sturckow said the crew would put on their orange spacesuits for landing.

"That sounds great, Houston," Sturckow said. "We're going to suit up then."

Among the crew returning to Earth was astronaut Sunita "Sunni" Williams, who spent 195 days on the space station, a women's endurance record.

The crew had three chances Friday to land at Edwards, the last at 6:59 p.m. ET. If the weather had spoiled all those opportunities, mission managers would have tried again Saturday, with another backup landing site in New Mexico in the lineup.

The preferred landing site is Kennedy, where it is easier and far cheaper to get Atlantis to its hangar to be prepared for its next mission in December.

Lands in California means it will cost $1.7 million and take up to 10 days to get it home to Florida aboard a jumbo jet.

Atlantis had enough power for its systems to orbit until Sunday, but managers didn't want to wait that long. The flight would have been extended to Sunday only if technical problems needed to be fixed.

During Atlantis' two chances to land Thursday, showers were within 34 miles of the landing strip at Kennedy Space Center, and clouds hung below an altitude of 8,000 feet, both violations of flight rules.

During the crew's 14-day mission to the international space station, the astronauts installed a new truss segment, unfurled a new pair of power-generating solar arrays and activated a rotating joint that allows the new solar arrays to track the sun.

Originally scheduled for 11 days, the mission was extended by two days to give astronauts time to repair a thermal blanket that had peeled away during the June 8 launch. Astronaut Danny Olivas stapled it back into place during a spacewalk. Another extra day in orbit was added after the weather in Florida prevented a landing Thursday.

The shuttle's visit to the space station was complicated by the crash of Russian computers that control orientation and oxygen production.

Both have seen big growth. Starting from zero about eight years ago, swapalease now brokers about 7,000 deals a year, according to Scot Hall, a company executive.

LeaseTrader is a bit older and larger, on track to handle about 35,000 transactions this year, according to LeaseTrader President Sergio Stiberman.

Stiberman said he has seen a 30 percent increase in people looking to sell their SUV leases this year.

"They definitely mention gas prices as a reason for getting out," said LeaseTrader spokesman John Sternal.

The fees are similar at both sites.

LeaseTrader charges $79 to sellers to post an ad; buyers pay $40 for a 60-day membership. The site runs credit checks so sellers know buyers can afford to assume their lease payments. If a transaction takes place, each party has to pay a $149 fee to the site.

Swapalease has a listing fee ranging from $49 to $149, depending on the amount of exposure the seller wants the ad to have. Membership fees for buyers run about $40. A transaction fee is only charged if a seller has the cheapest listing fee, in which case it's $95.

There are other costs too, noted Alex Rosten, an industry analyst at the auto research site Edmunds.com.

The leasing company usually charges a $200 to $400 fee to switch the names. And if the car isn't purchased when the lease runs out, there's often another $200 to $600 charge. If the car isn't local and needs to be shipped, that could run another $500.

"It's not as simple as signing on the dotted line," said Rosten. "Always research the fees before you pull the trigger."

For people looking to sell a lease, they'll have the best chance if the vehicle is still far from the mileage limit in the lease contract and the original lease terms are financially attractive.

Many of the ads feature cars that have used up all their allotted miles or where the original lease was expensive.

In those cases, the seller will usually try to sweeten the deal by offering a cash incentive, often thousands of dollars, to the prospective lease buyer.

Getting a good deal on these sites is possible, but it depends entirely on the details of the lease and the incentives being offered.

Sites like these can be good places to shop for a short term lease, say a year or less, said Rosten, for the simple reason that it's hard to find new car leases that short.

You can also benefit from someone's mistake, said Rosten. Some people still make big down payments on leased vehicles - something experts advise against - which can reduce the monthly cost substantially.

"If that happens, you can end up with a real bargain," he said.

But he offered one more word of caution: some of the ads on those sites are put there by dealers or leasing companies, not legitimate third parties.

"You're going there to take over a lease, not start a new one," he said. 

In a summer set for domination by inflated franchise movies in exhausting (and often exhausted) mega-mode, "Knocked Up" is a designated sleeper, the little movie that could -- and should -- clean up.

For once, critics and moviegoers are likely to be on the same page. Who isn't ready for a refreshingly frank, funny odd-couple comedy with engaging leads and too many belly laughs to count? (Lemme guess: I had you at "refreshingly.") Indeed, who isn't up for a movie that doesn't come encumbered with so much as a digit in the title, or even a colon?

OK, so "Knocked Up" sits comfortably on the same well-worn couch as "The 40-Year-Old Virgin," the previous hit from writer-director-producer Judd Apatow. Several of the same actors crop up, many of them veterans from Apatow's short-lived but fondly remembered small-screen gems, "Freaks and Geeks" and "Undeclared." But in this case, those are signs of high quality.

First and foremost there's Seth Rogen, an amiable, beer-bellied slob with a tight mop of curly hair, permanent five o'clock shadow and a voice like Baloo the bear, who seemed likely to play sidekicks for the foreseeable future.

Rogen's obvious limitations in the romantic leading man department aren't just the butt of the humor here, they're also the plot. It's "Beauty and the Beast," and he's the beast: his character, Ben Stone, is a twentysomething non-achiever living with four likeminded slackers. Their only employment is idle research into movie-star nudity for a proposed Internet start-up.

Beauty comes in the form of Alison Scott (Katherine Heigl from "Grey's Anatomy"), a perky blonde E! Entertainment presenter. Attractive and goal-oriented, she's on the fast track to success -- that is, until she has the misfortune to cross paths with Ben. Letting her hair down to celebrate a promotion, Alison is charmed by his gallantry at the bar, and then tipsy enough to take him to bed.

"You're prettier than I am," he acknowledges in a fleeting moment of bemused revelation as their clothes come off. But it's not until the morning after that she appreciates how far she's strayed from type, and another couple months before the consequences of this otherwise forgettable one-night stand come home to roost.

She's adamant about having the baby, and he wants to do the right thing. First, though, the parents will have to get to know each other and find out if they can fall in like.

None of this is exactly unexpected, and as the movie gestates the pregnancy's ballooning crises feel rather familiar, from finding the right obstetrician to the inevitable push-comes-to-shove about-face on an epidural.

But if this material has been thoroughly homogenized by decades of situation comedy, it's a tribute to Apatow that even the most hackneyed scenes get a new lease on life from his ruder instincts, steeped as they are in the vulgar, anarchic energies of adolescent angst.

In this movie -- as in "The 40-Year-Old Virgin" -- we can sense the "American Pie" generation growing up and settling down, but only with the very greatest reluctance. Ben and his buddies' slacker lifestyle isn't what you would call hygienic, but Apatow pictures it with a fond indulgence -- a note less evident in the barbed E! scenes featuring allegedly adult careerists.

Amid the candid jokes about pregnant sex, the earthy language and promiscuous drug use, "Knocked Up" is also surprisingly grown-up, a fundamentally honest, family-affirming picture in which even Alison's sister's severely strained marriage proves eminently salvageable. Leslie Mann and Paul Rudd contribute some of the movie's sharpest, funniest asides as the unhappy couple -- though in truth, the entire extended cast delivers the goods right down the line.

Even if, like most pregnancies, it goes on too long, "Knocked Up's" winning combination of benevolence and belly laughs will leave audiences smiling. In a nice touch, the end credits are illustrated with the cast and crew's baby pictures. And in Seth Rogen, for sure, a new star is born.