A Chinese Aids activist who was planning a commemoration of the 4 June 1989 Tiananmen Square killings has been detained.
The activist, Hu Jia, has not been seen since Saturday morning, when he was taken away for questioning by police.
His family said he sent text messages to friends saying he had been detained.
There is official unease ahead of the 15th anniversary of the Tiananmen events, when security forces killed hundreds of unarmed protesters.
A BBC correspondent in Beijing says Hu Jia is an outspoken advocate for people with HIV-Aids. he has publicly criticised the authorities on their treatment of Aids victims and other issues.
In an article written last month, he said the government should acknowledge that its killing of unarmed civilians in the 1989 protests was a serious mistake.
1989 TIANANMEN EVENTS
15 April - reformist leader Hu Yaobang dies
22 April - Hu's memorial service, thousands call for faster reforms
13 May - Students begin hunger strike as power struggle grips Communist party
15 May - Soviet leader Mikhail Gorbachev visits China
19 May - Zhao Ziyang (pictured) makes tearful appeal to students to leave
20 May - martial law declared in Beijing
3-4 June - Security forces clear the square, killing hundreds
He laid out plans to stage a vigil in Tiananmen Square on the night of 3 June to remember those who died.
His detention comes a week after three female activists who lost relatives in 1989 were held in custody for five days.
It is a sign the authorities are moving early to head off trouble ahead of this sensitive anniversary.
A new generation of leaders has come into power since the killings, but they have shown no sign of a change in stance and have defended the behaviour of their predecessors, our correspondent says.
Mr Hu's detention came as foreign media reported that Zhao Ziyang, the Communist Party general secretary sacked for his handling of the Tiananmen Square protests, had recently been seriously ill.
Mr Zhao, 84, has not been seen in public since he appealed to students to leave the square peacefully soon before martial law was announced.
According to the press reports, Mr Zhao was in hospital in February but has since been allowed to return home.
Mr Zhao is best remembered inside China for his commitment to reform and cracking down on official corruption - the same attributes that protesters admired in his predecessor, Hu Yaobang, whose death in 1989 triggered the Tiananmen Square events.