Jiang Tianyong, 46, sat in court in the southern city of Changsha as he made his confession in what rights group Amnesty International called a 'sham trial'.
Jiang had taken on many high-profile cases, including those of Falun Gong practitioners, Tibetan protesters and victims of the 2008 contaminated milk powder scandal before being disbarred in 2009.
The family has not had contact with Jiang since his sudden disappearance on November 21 en route from Beijing to Changsha, where he had gone to enquire about detained human rights lawyer Xie Yang.
Xie was detained in the "709 crackdown" of July 2015, and his claims of being tortured in custody, which Jiang helped to publicise, had drawn international concern.
Authorities in that crackdown detained more than 200 people, including lawyers who took on civil rights cases considered sensitive by the ruling Communist Party.
Xie was found guilty of "subversion of state power" and released on bail in May after what critics described as a show trial.
Jiang told the court on Tuesday that he "deliberately fabricated torture details of Xie Yang while he was in police detention and played to western media's taste, aiming to tarnish the image of the government".
"I know what I did was wrong, it is a serious crime," Jiang said.
Video clips of the trial were posted on the official microblog of the Changsha Intermediate People's Court.
Amnesty International China researcher William Nee said Jiang's treatment "epitomises many of the worrying aspects of the lawyers crackdown".
Nee said this included the "harassment of family members, not letting the accused access their lawyer, prosecution based on charges that don't comply with international standards, blocking the public from attending, all while presenting the trial as real on social media".
Jiang's arrest is part of a tightening of controls on civil society that began in 2012, when President Xi Jinping took power.
While the government initially targeted political activists and human rights campaigners, it has increasingly turned its attention to the lawyers who represent them, closing avenues for legal activism that had opened up in recent years.
