Chinese authorities tried last week to detain an American human rights activist at an international conference in Beijing, prompting a tense standoff that ended after state security agents interrogated the woman in front of U.S. diplomats and two European ambassadors for more than an hour.

The European Union has lodged a protest with the Chinese government over the incident, which occurred June 21 and disrupted a two-day seminar on human rights. The activist, Sharon Hom, had been invited by China to the conference organized by the E.U. and the Chinese Foreign Ministry, according to Hom and diplomats familiar with what happened.

The government's decision to allow Hom to participate in the seminar appeared at first to be a diplomatic breakthrough because she is executive director of Human Rights in China, a New York-based advocacy group that the governing Communist Party has repeatedly condemned. The Beijing government also agreed to host representatives of Amnesty International and Article 19, a London group that campaigns for freedom of speech.

But during the banquet on the last night of the conference, plainclothes agents of the Ministry of State Security stopped Hom outside her hotel room and demanded she leave with them. Even after the protests of U.S. diplomats, the E.U. ambassador to China and its ambassador for human rights, the agents insisted on questioning her, Hom said.

The episode is the latest sign that Chinese President Hu Jintao has granted the country's security services greater authority to act against perceived threats to the party's grip on power. In recent months, police have detained several mainland dissidents and activists, as well as a prominent Hong Kong journalist employed by a Singapore newspaper. Propaganda officials have also ordered a general tightening of controls on the Internet and in state media.

The incident also suggests there may be a rift within the Chinese government about how to deal with pressure from the United States and Europe for progress on human rights issues and the release of political prisoners. Hom said Chinese diplomats and other officials treated her with respect during the seminar but none came to her defense when the state security agents arrived.

"One of the lessons of this incident is that while you can engage up to a point with the more progressive elements in the government, sooner or later you have to deal with the reality of the state security apparatus and the fact it can trump the others," Hom said by phone from New York. "One way to do that is to not to be intimidated and to insist on the inclusion of civil society and independent voices in China when engaging the government."

A spokeswoman for the U.S. Embassy and another for the State Department said they were unable to immediately comment on the incident. E.U. diplomats in Beijing declined to comment, and the Chinese Foreign Ministry also did not respond to questions.

Hom said she waited until Friday to go public because the E.U. filed strong protests with the Chinese and she wanted time to analyze the situation carefully.

She said the incident began when she left the banquet early and returned to her hotel room, where she found that her key card no longer worked. Suddenly, two men and a woman surrounded her, identified themselves as state security agents and said they wanted to ask her a few questions, she said.

Hom said she agreed to go downstairs with them and offered to answer their questions in the lobby. But the agents insisted she go with them to a nearby location where it would be "more convenient" to talk. They pressed her to go for nearly 20 minutes, and their demands grew more threatening, Hom said.

One of eight agents in the lobby grabbed her arm when she began walking away, Hom said, but she made it back to the banquet and requested help from European diplomats there.

After a stalemate that lasted two hours, the agents eventually agreed to interrogate Hom in the presence of the U.S. deputy chief of mission and the two European ambassadors, she said. For more than an hour, she answered questions about her identity, her participation in the seminar, her activities in Beijing and her opinion of the changes in China over the past two decades.

Hom, who was born in Hong Kong, said one of the agents argued that though she is an American citizen, she is also "still Chinese." He then implied that she therefore could participate only in organizations approved under Chinese law, she said.

The interrogation ended shortly before midnight, Hom said, after she complained she was tired.

Hom said she was unsure why the agents targeted her. She said she did not meet with any Chinese outside the seminar. She also said she had visited China twice before while in her current job without a problem, as a tourist and a visiting law professor.