From the streets of Singapore to Trafalgar Square, jubilant Londoners celebrated their upset victory Wednesday over Paris for the right to stage the 2012 Summer Olympic Games.

Thousands who had crammed into Trafalgar -- built to honor a sea battle that marked another famous triumph over the French 200 years ago -- erupted with joy and disbelief when the president of the International Olympic Committee announced the results in a live broadcast from Singapore, seven time zones away.

Simultaneously, about 210 miles to the south, Parisians who had gathered at their City Hall anticipating success reacted with a collective groan. Some wept openly, while others drifted away in stunned silence as rain began to fall.

Madrid, Moscow and New York also were in the running at the start of what became the tightest site selection contest in modern Olympic history. They were eliminated in the first three rounds of committee voting, in which the lowest-scoring city was dropped each time. London won the fourth round by 54 to 50.

"We always knew that this was going to be a very tense result," Princess Anne, an IOC member and director of the London 2012 bid, said at a news conference. "You can't have five cities of that quality without it being very close," added the onetime Olympic equestrian.

Paris had lost two other recent bids for the Olympics and last hosted the Games in 1924. London staged the Olympics in 1908 and 1948. Britain had mounted three recent unsuccessful bids, twice by Manchester and once by Birmingham.

Paris had been considered the front-runner throughout the process, with a well-developed plan, an already-built stadium and an air of quiet confidence. French President Jacques Chirac had flown to Singapore on Tuesday for a quick appearance to make the final presentation and bask in the expected glory.

But London's bid, led at the Singapore selection committee meeting by two-time Olympic gold-medal runner Sebastian Coe, had gathered momentum in recent weeks. British proponents emphasized that the Games could help regenerate their capital's once-neglected East End area. Prime Minister Tony Blair spent three days lobbying officials in Singapore, emphasizing his government's commitment to the project, before returning home to chair the Group of Eight summit of world leaders in Scotland that began Wednesday.

Blair, grinning broadly, told reporters he had been too nervous to watch the televised selection ceremony and had paced the grounds of his hotel until he got a phone call from his London office with the surprising news. "It's not often in this job that you punch the air and do a little jig and embrace the person standing next to you," he said. "It's a fantastic thing. I'm absolutely thrilled."

For Blair, winning the Olympics is the latest in a series of triumphs. The prime minister's governing Labor Party won a third consecutive electoral victory in May, albeit by a smaller margin than in landslides of the past. Last week, Britain took over the presidency of the European Union for the next six months, and Blair promised to promote economic reform. London also was the flagship site for Saturday's worldwide Live 8 rock concerts. Blair, meanwhile, is pressing increased aid for Africa and renewed focus on global climate change at the G-8 summit.

In contrast, the losing Paris bid represented a new public humiliation for Chirac, Blair's long-standing rival for leadership of Europe. About a month ago, Chirac suffered a defeat when French voters rejected a proposed E.U. constitution he had championed.

Chirac, who had been quoted by a French newspaper over the weekend as privately mocking British cuisine and trustworthiness, issued a statement congratulating London.

But Paris Mayor Bertrand Delanoe expressed the French team's sense of bitter frustration, accusing Blair and the British of violating the Olympic spirit with aggressive lobbying tactics. "The reason we lost is because we stuck to fair play," he told the French television news channel LCI. "We didn't use the weapons of war that others did. Over the past two days when I've been asleep in my bed, Tony Blair has been giving hundreds of interviews throughout the evening."

Blair and Chirac led glamour-studded teams to Singapore. The British contingent included soccer star and European heartthrob David Beckham, who comes from the East End. The French brought along their own soccer superstar, Laurent Blanc, plus film director Luc Bresson and retired skiing gold medalist Jean-Claude Killy. Muhammad Ali, former secretary of state Henry A. Kissinger and Sen. Hillary Rodham Clinton (D-N.Y.) came to support New York's bid.

But 30 London schoolchildren -- brought here to emphasis the city's commitment to young people -- may have been the more influential stars. Olegario Vazquez Rana, an IOC member from Mexico City, switched his vote to London after New York was cut, saying he was most impressed by London's "support for the children, for the future."

London now must deliver on plans to build a new stadium and Olympic village, as well as a mass transit network to support it. The cost is estimated at nearly $2.5 billion, according to the city's mayor, Ken Livingstone. Not all of the land has been purchased, and some East Enders have objected to losing their shops or houses to the project. In its evaluation report, the IOC's evaluation commission praised the "very high quality" of London's bid but warned that "given the magnitude of the project, careful planning would be required to ensure that all facilities are completed on time."

None of that seemed to matter Wednesday night as Londoners celebrated in Stratford, the East End neighborhood at the center of the project, and throughout the city.

"This will give a real boost to the regeneration of the East End," said Nigel Phipps, an investor service official who was returning from Paris on the Eurostar train. "It also adds to the feeling of renewal all over London."

When London last hosted the Olympics in 1948, the country was still using ration books from World War II, and the Games helped the city recover its sense of optimism. Sophie Edwards, a hospital physician who lives in Leytonstone, one of the East End neighborhoods expected to benefit, said she believed the new Games would succeed in "making people feel good about themselves."

But in this true-life tale of two cities, Parisians were not impressed. "Well, you have to be a good sport about it, but I thought Paris would win for sure," said Julia Mery, a graphics designer.

"What annoys me most is that we lost to the British again," said Nino Bigudi, a Paris bar owner. " I don't really mind about the Games -- I'm more interested in rugby -- but what I hate is that the English won."

Frankel reported from London and Paris; Nakashima reported from Singapore. Researcher Andrea Denham in Paris contributed to this report.

Workers roll up a carpet made to look like a running track that had been laid out in front of Paris City Hall before the surprising vote by the International Olympic Committee in Singapore.