Activists and government critics in China have called for the release of a prominent human rights lawyer detained by authorities.
Police announced on Friday that they had arrested Gao Zhisheng "for suspected involvement in criminal activities", without specifying any charges.
Gao's previous clients have included disgruntled private oil investors, rural protesters and members of Falun Gong, the spiritual group that is outlawed in China.
A petition signed by 40 Chinese activists and dissidents said that Gao "has fully exercised his right to engage in civic rights defence activities and his freedom to express his political views".
"These rights should be respected and protected by the government."
Signatories of the petition circulated by e-mail included Yu Jie, a Christian critic of the ruling Chinese Communist Party who recently met George Bush, the US president, at the White House and Ding Zilin, an academic who has sought redress for families of victims of the 1989 crackdown on pro-democracy protests.
The petitioners said Gao would now become a focus for Chinese "rights defenders" aiming to fight what they see as increasing government repression.
"Now he's become the most important case for all of us," Beijing-based activist Hu Jia said of Gao. "He's been the most prominent rights defender for a long time."
Gao had often predicted his campaigns could cause authorities to arrest him and late last year Beijing authorities suspended his office licence for a year.
The announcement of Gaoâ€™s arrest came the same day as China tried a blind human rights activist, Chen Guangcheng, in the eastern province of Shandong, having earlier detained his main defence attorney until the hearing ended.