The charge against Xu, 40, a lawyer and scholar who has been under detention since July, is "assembling a crowd to disrupt order in a public place."
The trials of at least six other activists are scheduled to follow.
Xu's lawyer, Zhang Qingfang, recently told the New York Times Xu is determined to fight the case against him, although he believes his conviction, which could send him to prison for up to five years, is almost certain.
He and other activists are among those who have been advocating such issues as greater rule of law and fairer access to education, the Los Angeles Times reported.
The U.S. newspaper said there had been hopes China's new leadership under President Xi Jinping would support more free speech in the country. But a media blackout on Xu's case has prevented information reaching most people on the mainland.
The Times quoted prosecutors as alleging Xu and others organized and incited hundreds of people to gather in front of government buildings and at train stations and universities in Beijing in 2012 and 2013 and distributed leaflets calling for education reform and asset disclosure for government officials.
"The security people have made the judgment that they could be facing serious demonstrations. This is a pre-emptive strike ... . They want to take out the leaders and organizers, and send a clear signal that no one is allowed to organize street protests," Feng Chongyi, a professor of China studies at the University of Technology in Sydney, told the newspaper.
Xu has said his New Citizens Movement seeks to open "a new path for the Chinese nation, a path toward liberty, justice and love." In a 2012 essay, he reportedly described it as a political movement in which China "bids utter farewell to authoritarianism and completes the civilized transformation to constitutional governance," and as a social movement to "completely destroy the privileges of corruption, the abuse of power, the gap between rich and poor, and to construct a new order of fairness and justice."
His lawyer was quoted as saying during a visit to Xu Tuesday, he found his client calm. The lawyer said Xu's wife, Cui Zheng, who recently gave birth, told him the trial at least would let her see her husband for the first time in more than six months.
Xu's trial would be followed by trials of fellow activists in Beijing and the southern city of Guangzhou.
Xu's lawyer said prosecutors would only permit their witnesses' statements to be entered into the official record, while the defense's efforts to bring five witnesses had been rejected.
"This whole trial is being conducted in a black box, where we cannot actually see the entire process," he said. "I always hope and strive to represent my clients in a fair trial, but this is not fair, so I will keep silent."
The New York Times quoted Xu's supporters as saying the trial will demonstrate the determination of Xi and other Communist Party leaders to extinguish any organized opposition, however mild, challenging their control.