Hundreds, perhaps thousands, died when China sent in the army
China has blocked several websites ahead of the 20th anniversary of the suppression of the Tiananmen protests.
Chinese internet users were unable to connect to the social networking service Twitter, their Hotmail accounts and the photo-sharing service Flickr.
Meanwhile veteran dissident Wu Gaoxing, who was jailed for his part in the 1989 pro-democracy movement, was held at the weekend, a fellow activist said.
China bans discussion of the events in and around Tiananmen Square in 1989.
Thursday 4 June is the 20th anniversary of the crackdown, when troops quelled weeks of protest by students and workers.
China has never released a death toll from the suppression on what it says was a counter-revolutionary conspiracy. Hundreds are believed to have died in and around the square.
The BBC's James Reynolds in Beijing says that as the anniversary of what China calls the "4 June incident" gets closer, the Communist Party appears to be in a particularly vigilant mood - it wants to make sure that there is no mention of the subject whatsoever.
Access to Twitter was denied shortly after 1700 (0900 GMT) on Tuesday.
Meanwhile former Tiananmen detainee Chen Longde told AFP news agency that Wu Gaoxing had been picked up by police on Saturday in the eastern city of Taizhou.
Mr Wu had signed a letter to President Hu Jintao seeking compensation for those jailed after troops shot protesters on 4 June 1989.
The letter said former prisoners are unable to find steady jobs and are deprived of medical benefits and pensions.
Mr Wu was detained for two years after protesting in 1989 in the eastern province of Zhejiang, as pro-democracy demonstrations were taking place in Beijing.
The human-rights group Amnesty International says as many as 200 people remain in detention for their involvement in the 1989 pro-democracy protests.