Padma-Dolma Fielitz, a 21-year-old Tibetan woman from Germany, and another activist held Tibet's national flag aloft just outside the south entrance of Tiananmen Square, according to a statement by Students for a Free Tibet.
Photos on the group's Web site show a woman identified as Padma-Dolma being dragged on the ground as police and plainclothes agents try to wrest the flag away from her.
Shortly afterward, three other activists tried to unveil a banner that said "Tibetans are dying for freedom" but were stopped by authorities, the group said.
All five protesters - including two Americans and two Canadians - belong to the New York-based group.
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.
Pro-Tibet activists say China is using the Olympics to legitimize its rule in Tibet and have used the games as a platform to spotlight their cause.
The demonstration lasted about five minutes and all five were taken away, the group said. It was the latest in string of similar protests in the past week as Beijing became a focus of international attention because of the Olympics, which opened Friday.
Hours earlier, Hua Huilin said he and his brother, Hua Huiqi, a housing activist and pastor in an underground Christian church, were stopped by two black cars while bicycling to the church attended by Bush.
Hua Huilin said they were taken away in separate cars by security agents, whom his brother recognized from previous encounters. He said they took away his brother's Bible and cell phone. He was released in the afternoon but Hua Huiqi was still at an undisclosed location, he said.
"I told him not to go because it's during the Olympic Games and this period is sensitive," Hua Huilin told The Associated Press in a telephone interview. "But he was determined to go because he said that church was where he was baptized. So I went with him hoping to protect him."
The line was disconnected three times during Hua's conversation, a sign that authorities were monitoring the telephone.
Hua Huiqi had been planning for days to be at Kuan Jie Protestant Church at the same time as Bush, who is in Beijing for a few days to see some Olympic events and meet with China's leaders. It was not immediately clear what Hua had planned to do at the church.
A man who answered the telephone at the Beijing Public Security Bureau's spokesman's office said they would try to find out what happened. He would not comment otherwise or give his name, as is common with Chinese officials.
Chinese authorities often take activists away before and during sensitive periods. They have tightened already stringent restrictions to curb potential criticism or protests during the Olympics.
On Saturday, the organization Chinese Human Rights Defenders said the wife of jailed activist Zeng Jinyan disappeared Thursday and may have been taken by police to prevent her from speaking to journalists during the games.
There have been small protests by foreign activists near the National Stadium, a key Olympics venue, and Tiananmen Square in the past week. They have ended peacefully and no arrests were reported, though several people have been deported.
Hua Huiqi, a pastor who fought against a development project in his neighborhood, has been arrested and beaten several times over the last few years because of his religious activities. He served six months in jail for "obstructing official business."
Rights groups say that charge stemmed from an incident in which Hua and his mother scuffled with police as they prepared to deliver a petition to the central government over the demolition of their home in 2001.
China allows worship only in officially approved churches such as the one Bush visited Sunday with first lady Laura Bush, so millions of people pray privately in house churches to avoid detection.
After attending the service on Sunday, Bush gathered for photos with parishioners on the front steps and told reporters that no country should "fear the influence" of religious freedom, a clear reference to China's tight control of churches.
"It just goes to show that God is universal," Bush said. "No state, man or woman should fear the influence of loving religion."