It also diminishes hopes for meaningful political change, even as China pledges to embark on economic reforms.
Separately, one of China's most prominent dissidents, Hu Jia, who frequently accuses authorities of infringing civil liberties, said police had summoned him on a charge of "suspicion of causing a disturbance".
The Beijing No. 1 Intermediate People's Court found Xu guilty of "gathering a crowd to disturb public order", the court said on its official microblog. Xu was tried on Wednesday.
Xu's lawyer Zhang Qingfang said he would meet Xu within the next two days to determine whether to lodge an appeal.
"He said (in court) that the last remaining dignity of the Chinese legal system has been destroyed," Zhang told reporters.
"It's not that we can't bear this result but that, fundamentally, the guilty conviction is illegal, is unreasonable and unfair."
The United States condemned the move.