A group of protesters pleaded for help Tuesday from President Barack Obama before his visit to China next week, saying anyone seen as a troublemaker is often treated harshly before major events in the capital.

Chinese authorities regularly crack down on dissidents before politically sensitive anniversaries or visits from foreign dignitaries, but the protesters said ordinary people get caught up in the sweeps.

"We are here because Obama is the president of a free and democratic country, he is coming to China, therefore the Chinese government will put pressure on us, the same way it happens on special dates," said Yang Qiuyu, a housing rights activist who was among the 30 people protesting in a central Beijing park.

The protesters want Obama to ask his Chinese hosts why such crackdowns must occur.

The protesters had come to Beijing with housing complaints _ demolished homes, for example _ but were drawn into demanding human rights after getting what they said was unfair treatment from authorities.

One woman flashed a "V for victory" sign at an Associated Press Television News camera and opened her black jacket to reveal a white shirt with the handwritten phrase "I want human rights." Police quickly took her away.

"We are being detained illegally or being put under house arrest, our freedom is limited," Yang said.

Police arrived shortly after the protest started and took a number of people away.

"Why you detaining me? Do you have a formal accusation? I am being kidnapped!" Yang said as he was pushed into a police van.

Police asked the other protesters to leave the park.

Yang could not be reached Tuesday evening because his mobile phone was off. "It's been 8 hours since he was taken away," his wife, Wang Yuqin, said in a phone interview. She said she was at the police station and was not allowed to see her husband.

Other appeals are emerging before Obama's arrival Sunday, including a letter by the China Support Network, a U.S.-based rights organization, which wants Obama to help secure the release of several jailed lawyers and scholars.