Xie Yang, who had worked on numerous cases considered politically sensitive by the ruling Communist Party, was among hundreds of legal staff and activists detained in a crackdown in the summer of 2015.
Dozens of supporters and several diplomats gathered at the court in the central city of Changsha for the start of Xie's trial -- believed to be on charges of "inciting subversion of state power" -- were told the case would not be heard today.
A new date was not provided.
Xie, who was arrested in the so-called "709 crackdown", claims police have used "sleep deprivation, long interrogations, beatings, death threats, humiliations" on him, and the EU has voiced concern over his case.
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"Our country is always emphasising the rule of law, so through Xie Yang's case we can tell whether or not there is any kind of reality in that," a supporter at the courthouse told AFP, requesting anonymity.
"We are following this case very closely and our human rights counsellor is in Changsha today," a Beijing-based EU spokesman told AFP.
Xie is being represented by a court-designated advocate after he was denied the right to pick his own defence, his former attorney Chen Jiangang said.
The new lawyer, He Xiaodian, could not be reached for comment and the courtroom also did not answer calls.
Chen said He had contacted Xie's family on Monday night to advise them not to go to the courtroom. "But they did not get any clear information saying the trial date was different," he said.
Chinese President Xi Jinping has overseen a tightening of controls on civil society since assuming power in 2012, closing avenues for legal activism that had opened up in recent years.
While the government initially targeted political activists and human rights campaigners, it has increasingly turned its attention to the legal professionals who represent them.