Xu Jue, the mother of student protester Wu Xiangdong who was killed during the Tiananmen crackdown 20 years ago, has been prevented by police from visiting her son's grave to mourn his death, a human rights watchdog said Thursday.

The Information Center for Human Rights and Democracy quoted a statement by Xu as saying that she was caught by police Wednesday night when trying to sneak out of her house to go to the grave, and police presence around her house has been beefed up since then.

She said she believed police would take her to somewhere else for detention Thursday along with other members of Tiananmen Mothers, a group set up by some 200 parents of the dead students.

"In over 20 years, the authorities never revealed what really happened nor talked to the victims' families," Xu said in the statement. "Instead, they have been restricting personal freedom of citizens in violation of the Constitution. We therefore strongly condemn the authorities."

Hong Kong's Cable TV reported that families arriving at a public cemetery in Beijing where some of the slain students were buried were soon evicted by police after they drew media attention.

Another human rights advocacy group, Chinese Human Rights Defenders, listed 65 activists, including Xu Jue, who have been subjected to harassment from officials in order to prevent them from organizing or taking part in activities to commemorate the Tiananmen massacre.

"These individuals have been taken into police custody, had their movements restricted, been forced to leave their homes, or otherwise threatened or monitored by police," it said.

It said individuals subjected to "soft detention" are guarded by police stationed at their homes.

"Though individuals may be allowed to leave their homes during soft detention, they are closely followed and monitored by police or asked to travel in police vehicles, and often barred from meeting other 'sensitive' individuals," it said.

Meanwhile, authorities have shut down nearly 160 websites, ostensibly for "system maintenance," to prevent netizens from mobilizing online and from learning about commemoration activities planned in many cities around the world, the group said.

Beijing authorities have tightened monitoring of the former protesters, the victims' families and the media during the 20th anniversary of the bloody military crackdown.

The Information Center for Human Rights and Democracy has said Chai Ling, who is No. 4 on China's 21 most-wanted list, could be appearing in Hong Kong Thursday. Another then student leader Wuer Kaixi was deported Thursday from Macao after he came to turn himself up to the Chinese authorities Wednesday.

The center also said as many as 2,700 people could have been killed during the crackdown while 15,000 "prisoners of conscience" were convicted afterward, including students, workers and citizens, who some of them still in prison.