Chinese security forces blanketed Tiananmen Square on Wednesday ahead of the 20th anniversary of the June 4 crackdown on pro-democracy protesters, while Twitter and Hotmail Internet services in China were blocked.

Black police vans lurked at the side of the Forbidden City, while police and paramilitary forces patrolled through crowds of tourists enjoying a sunny summer morning.

Tanks rolled into Tiananmen Square before dawn on June 4, 1989 to crush weeks of student and worker protests. The ruling Communist Party, which has never released a death toll, fears that any commemoration of the crackdown could challenge its continuing hold on power.

In a sign of nervousness, China on Tuesday blocked access to popular Internet services Twitter, online photo sharing service Flickr, and email provider Hotmail. Foreign newscasts about the anniversary were also cut.

Police prevented at least four foreign television crews from filming on Tiananmen Square in the week before the anniversary.

Administrators at Chinese universities have been told to keep a close eye on foreigners in their departments. Taxi drivers were instructed to watch out for suspicious passengers, especially those headed toward Tiananmen Square.