One athlete, U.S. swim star Amanda Beard, also made a public political gesture, on behalf of animal rights.
All of the groups had problems with Chinese authorities, who are determined to make sure the communist government's plan for the Beijing Games to be an international showcase for the country goes off without a hitch.
Meanwhile, U.S. President George W. Bush plans to pointedly express "deep concerns" about the state of human rights in China and urge the communist nation to allow political freedoms for its citizens.
"America stands in firm opposition to China's detention of political dissidents, human rights advocates and religious activists," Mr. Bush is to say in the marquee speech of his three-nation Asia trip. "We speak out for a free press, freedom of assembly and labor rights - not to antagonize China's leaders, but because trusting its people with greater freedom is the only way for China to develop its full potential."
Mr. Bush is to deliver the address in a Bangkok, Thailand, convention center on Thursday morning to a crowd of foreign diplomats, Thai government leaders and business officials, before flying to China later that day. The White House released the text of the president's speech on Wednesday, nearly 18 hours in advance, as Mr. Bush traveled to Thailand from South Korea.
No arrests were reported despite the rare displays of dissent in the capital, where normally stringent controls over criticism of the government have been tightened even further for the 17-day Olympic competition.
Four foreign activists were led away by police after they hung pro-Tibet banners outside the Beijing National Stadium, where Friday's opening ceremony will be held.
Two men from Students for a Free Tibet each climbed a light pole in front of the so-called Bird's Nest and put up the banners at dawn, said Lhadon Tethong, the New York-based group's executive director. The other two - a man and a woman - provided support from the base of the poles, she said.
It was the first demonstration at a games venue. Beijing organizers condemned the protest.
"We express our strong opposition," said Sun Weide, spokesman for the Beijing Olympics organizing committee. "In terms of assembly and demonstrations, China has related laws and regulations. We hope that foreigners will respect the related Chinese laws and regulations."
Sun said the demonstrators were "persuaded to leave" by police, who received tips from local residents about the protest. The four have not been arrested or taken to a police station, he said.
International Olympic Committee spokeswoman Emmanuelle Moreau said organizers should expect people to "use the platform of the Olympic Games to draw attention to their causes."
"The IOC are confident Beijing city authorities will assess the situation reasonably and act with tact and understanding," she said.
Later Wednesday, three Americans spent almost an hour in the iconic Tiananmen Square criticizing Beijing's handling of issues ranging from forced abortions to the banned Falun Gong spiritual movement to pro-democracy demonstrations in 1989.
"It was important for us that there be a clear voice speaking out against the Chinese government's abuse of human rights," Rev. Patrick Mahoney, director of the Christian Defense Coalition in Washington, said in a telephone interview.
The trio also set up a banner in the square that said "Christ is King" and knelt and prayed. Brandi Swindell, national director of the activist group Generation Life, also put out seven roses in memory of those who died in the military crackdown on pro-democracy protests on and near the square in 1989.
They said plainclothes security agents and police officers tried to block the banner with umbrellas and started shoving the group when they tried to walk around the square. The agents eventually pushed them out of the area and made them sit nearby for almost an hour, checking their passports, before letting them go, Mahoney and Swindell said.
"It's so shocking being an American ... to see the blatant oppression," Swindell said.
Tibet has been an extremely sensitive topic since protests against almost 50 years of Chinese rule turned violent in the region in March. Many Tibetans insist they were an independent nation before communist troops invaded in 1950, while Beijing says the Himalayan region has been part of its territory for centuries.
The protests prompted a massive crackdown by Chinese security forces. Pro-Tibet groups say scores of monks and nuns have been arrested, imprisoned and beaten since March.
One of Wednesday's banners said "Tibet will be free" and "Tibet Freedom" in Chinese. The other read, "One World, One Dream" - the slogan for the Beijing Olympics - followed by "Free Tibet." One of the men also flew the flag of the Dalai Lama's former Tibetan government.
Tethong's group identified the protesters as Thom, 24, of Scotland; Lucy Marion, 23, of England; Phill Bartell, 34, of Bridgewater, New Jersey; and Tirian Mink, 32, of Portland, Oregon.
While Beijing has announced that it would allow applications for public protests in three designated areas, it isn't immediately clear if any had been accepted. None of the protests Wednesday were in the designated areas.
China's rights record and its policies in Tibet and Sudan have been a flash point in the run-up to Olympics.
Former Olympic speedskater Joey Cheek had his visa revoked by Chinese authorities Wednesday, hours before he was set to travel to Beijing to urge Beijing to help make peace in war-torn Darfur.
Beard, the reigning Olympic champion in the 200-meter breaststroke, on Wednesday unveiled a poster of herself naked in support of the anti-fur activist group People for the Ethical Treatment of Animals. The poster's launch was planned for a hotel but happened outside the Athlete's Village after Chinese authorities canceled the hotel event.