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Rio de Janeiro was the winner over Madrid in the final round of voting. The committee delivered an unexpectedly early knockout blow to Chicago, which was eliminated in the first round. Tokyo was ousted in the second.

Jacques Rogge, the president of the committee, made the announcement, sending crowds in Rio de Janeiro into celebration.

Tens of thousands of people began partying early in Rio on the Copacabana beach. Musicians played samba music from a main stage flanked by large screens, as people danced, held towering cones of cotton candy and showed off the national colors of Brazil by donning yellow-and-green wigs or yellow-and-green bikinis. A beach ball bounced above the crowd, marked with the words, “It’s Rio’s time.”

The scene was different earlier in Chicago as throngs in Daley Plaza gasped in disappointment when Rogge announced that Chicago was out. It was a surprisingly early exit, especially after President Obama’s whirlwind trip to boost the bid of his adopted city. Mr. Obama was the first American president to make an in-person appeal for a bid city and the first lady, Michelle Obama, had also come this week to lobby I.O.C. members for votes.

Chicago’s bid leaders had worked for nearly four years and spent close to $50 million to bring the Summer Olympics to the United States for the first time in 20 years. Chicago had been considered among Olympic insiders as a favorite to win the Games, along with Rio.

Instead, the I.O.C. delivered a crushing blow to American hopes for the second straight time. New York’s bid was eliminated in the second round of voting for the 2012 Olympics.

United States Olympic Committee leaders appeared stunned by the news and had no comment as they left the voting hall. Mr. Obama was flying back to Washington at the time of the vote.

President Luiz Inácio Lula da Silva of Brazil had led Rio’s emotional appeal to win the Games, and the urge to make history tugged at the I.O.C.

Juan Antonio Samaranch of Spain, a former I.O.C. president, had made a passionate and personal appeal for Madrid in its presentation. “Dear colleagues, I know I’m very near the end of my time, I’m 89 years old,” he said. “I ask you to consider granting my country the honor and also the duty to organize the Games and Paralympic Games in 2016.”

To win, Rio’s bid leaders had to overcome sentiment for Samaranch and concerns about security in the Brazilian city. There were also concerns that the country would be overextended because it is hosting the 2014 World Cup.

Chicago had plenty of its own hurdles, with many issues idling in what is often a strained relationship between the Eurocentric I.O.C. and the Olympic efforts in the United States. Chicago hoped to do better than New York did in the last bidding process, but recent turnover at the United States Olympic Committee and a scrapped idea to start an American Olympic network over I.O.C. objections did not help Chicago’s chances.

That made Mr. and Mrs. Obama’s visit critical. King Juan Carlos of Spain and Prime Minister Yukio Hatoyama of Japan also came to work on behalf of Madrid and Tokyo.

Voting was done electronically and by secret ballot. It was done in rounds until one city earned a majority of votes. I.O.C. members from the countries of the bid cities do not vote while those cities are still in contention. Chicago received the fewest votes in the first round. That fate befell Tokyo in the second round.

Teams from the four candidate cities delivered their final presentations to the 104-member I.O.C. and answered every lingering question about the strengths and weaknesses of their bids earlier Friday.

The 10-person Chicago bid team, led by Mr. and Mrs. Obama, put on a presentation heavy on emotion and visual images without getting too deep into the details of the bid.

“To host athletes and visitors from every corner of the globe is a high honor and a great responsibility,” said Mr. Obama, whose Chicago home is a short walk from the prospective Olympic Stadium. “And America is ready and eager to assume that sacred trust.”

In the official question-and-answer session after the Chicago presentation, Syed Shahid Ali, an I.O.C. member from Pakistan, asked the toughest question. He wondered how smooth it would be for foreigners to enter the United States for the Games because doing so can sometimes, he said, be “a rather harrowing experience.”

Mrs. Obama tapped the bid leader Patrick G. Ryan, so Mr. Obama could field that question.

“One of the legacies I want to see is a reminder that America at its best is open to the world,” he said, before adding that the White House and the State Department would make sure that all visitors would feel welcome.

Tokyo went next and tried to overcome impressions conveyed by I.O.C. evaluations that its bid was lacking. The bid team emphasized to the committee how environmentally friendly its plans are and the positive impact an Olympics would have on the youth of Japan.

“A lot of I.O.C. members suggested that we needed more passion and emotion,” the bid leader Dr. Ichiro Kono said. “We wanted to show that.”

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