A leading Chinese doctor who criticised the Communist Party's 1989 Tiananmen Square crackdown has disappeared on the eve of its 15th anniversary.
Police are quickly clamping down on attempts to mark the crackdown
Jiang Yanyong is one of several potential critics thought to have been taken from Beijing or put under house arrest ahead of the 4 June anniversary.
Rights groups have called on China to review its handling of the protests.
Hundreds of people were killed when troops and armed police opened fire on unarmed protesters around the square.
Jiang Yanyong was the doctor who first shed light on a government cover-up of the Sars virus by contradicting official figures for the spread of the deadly disease.
He caused further embarrassment when he wrote a letter to the country's top leaders in February 2004, asking them to admit mistakes in the handling of the 1989 Tiananmen protests.
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 makes tearful appeal to students to leave
20 May - Martial law declared in Beijing
3-4 June - Security forces clear the square, killing hundreds
Mr Jiang and his wife have not been seen in their Beijing home since early on 1 June, according to their daughter Jiang Rui, who is demanding an investigation.
"While we do not want to speculate as to what happened to
our parents, we believe the authorities of Beijing 301 Military
Hospital [the hospital where Dr Jiang works] are deliberately withholding information from us," she told Reuters news agency.
Mr Jiang and his wife are not the only people to have gone missing in the run-up to the anniversary.
Another prominent activist, Liu Xiaobo, could no longer be contacted and may also have been taken out of the capital, according to the Hong Kong-based Information Centre for Human Rights and Democracy.
Others have been reported missing, and phone lines of known activists have been cut.
Universities are being monitored to prevent commemorations taking place.
Ding Zilin, who heads the Tiananmen Mothers group representing those who lost children in the crackdown , told the French news agency AFP: "I'm under surveillance 24 hours a day. A car is parked right outside my home."
These moves show how anxious the government is ahead of anniversary, says a BBC correspondent in Beijing, Louisa Lim.
China has also reacted angrily to plans by the US Congress to condemn the suppression of the 1989 demonstrations.
"There are a handful of people in the United States Congress
that cannot stand what happens in China, and they are using all kinds
of pretexts to defame China," said foreign ministry spokesman Liu
The US Congress resolution condemns "the ongoing and egregious human rights abuses" in China, and urges the government to order an independent inquiry into the events 15 years ago.
The resolution is expected to be put to the vote in Washington on Thursday.
The human rights organisation Amnesty International has also called for an independent inquiry into the Tiananmen massacre.
In a statement to mark the anniversary, Amnesty said that those found responsible should be tried and brought to justice.
But our correspondent says that China's rhetoric and its moves to silence its critics indicate that there is virtually no chance it will change its official stance on the Tiananmen killings soon.
In fact, there are even reports that Chinese officials have been ordered to
watch a new documentary on the demonstrations, to persuade them that the crackdown was the only option available.
Government sources told Reuters news agency that the four-hour documentary was being shown to people holding the rank of ministry department director or higher.
"Young cadres need to watch it because many think the
crackdown was unnecessary," one source told Reuters.