CHINA locked down Tiananmen Square on the eve of the 20th anniversary of the crackdown on pro-democracy protests there, after earlier restricting access to Twitter, Hotmail and a range of other websites and blogs.

Several hundred police, paramilitaries and other security personnel swarmed over the square in the heart of Beijing, where the army crushed seven weeks of protests on the night of June 3-4, 1989.

Police examined visitors at security checkpoints dotted around the square, scrutinised bus passengers disembarking on nearby streets and checked the bags and papers of people approaching the square from surrounding neighbourhoods.

Foreign newspapers sold in Beijing in the past few days have had pages ripped out. Television screens in the diplomatic compound went black when the BBC was airing a report about Tiananmen Square, and came back on when reports switched to the missing Air France jet.

"They feel this is a fragile time for China," said the artist and activist Ai Weiwei, whose blog was shut down at the weekend.

Notwithstanding the two decades that have elapsed, the demonstrations of 1989 remain one of the most sensitive subjects in China. The very mention of the date June 4, when the People's Liberation Army moved in to crush the demonstrations, is banned in the Chinese press.

Police have also been detaining writers, activists and especially former dissidents who took part in the 1989 demonstrations.

Ding Zilin, whose 17-year-old son was killed in the protests and who heads the group Tiananmen Mothers, was told to leave Beijing until the anniversary is over, the New York-based group Human Rights in China said.

At least one former participant, Wu Gaoxing, has been taken into custody without explanation from his home in Zhejiang province. Mr Wu, who spent two years in prison for his role, was one of five authors of an open letter to the President, Hu Jintao, complaining of the continued mistreatment of participants.

The Government appears fearful that activists will stage some kind of demonstration to commemorate June 4, as is happening around the world.

Besides Twitter, which went down on Tuesday, the Chinese have blocked Microsoft's Hotmail and Bing and the photo-sharing site Flickr. YouTube has been blocked since early April.

Jeremy Goldkorn, an expert in online media in China, said he thought the Government was particularly upset that people were trading information about how to read a just-published controversial memoir. It was written by Zhao Ziyang, the former secretary-general of the Communist Party who was fired in 1989 for his opposition to a military crackdown.

Agence France-Presse, Los Angeles Times