BEIJING — A prominent Chinese rights advocate who was detained last month has spoken out in a video shot from behind bars at a detention center in Beijing, urging citizens to stand up against the government and turn their rights into reality.

The unusual act of defiance by the advocate, Xu Zhiyong, was available for viewing on a number of Web sites Thursday, including YouTube and Voice of America, after being disseminated by his supporters.

Handcuffed and wearing a neon orange jail vest, Mr. Xu, 40, spoke in the video for just over a minute to an unidentified visitor.

“I propose that we all act as citizens, upstanding citizens, and exercise the rights of citizens under the Constitution,” he said. “No matter what becomes of this society, however crushed or absurd, this country needs a group of brave citizens to stand up and abide by their convictions, turning their rights, responsibility and dreams into reality.”