BEIJING - Chinese authorities blocked popular Web sites like Twitter and Flickr on Tuesday after forcing dissidents from Beijing in a clampdown ahead of the 20th anniversary of the Tiananmen Square pro-democracy protests.

Exiled former student leader Chai Ling, meanwhile, issued a rare public statement before Thursday's anniversary of the bloody crackdown, calling for the release of political prisoners, an independent investigation into the events and permission for former student leaders to return home.

China has never allowed an independent investigation into the military's crushing of the protests, in which possibly thousands of students, activists and ordinary citizens were killed.

Government Internet monitors have shut down message boards on more than 6,000 Web sites affiliated with colleges and universities, apparently to head off any discussion of the 1989 events, the Hong Kong-based Information Center for Human Rights and Democracy said.

Numerous blogs maintained by government critics such as avant garde artist Ai Weiwei have been blocked and popular social media sites such as Twitter and photo-sharing site Flikr could not be accessed within China on Tuesday. Video-sharing site YouTube has been blocked for months.

Authorities have also been steadily tightening surveillance over China's dissident community ahead of this year's anniversary, with some leading writers already under house arrest for months.

Despite China's official silence, the crackdown remains a major topic for human rights groups and pro-democracy supporters in the Chinese-ruled Hong Kong autonomous region, where this year's June 4 vigil is expected to draw tens of thousands.

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