One of China's most popular Internet chatrooms said on Tuesday it was closing for 10 days, in a sign of jitters before the 10th anniversary today of the military crackdown on the Tiananmen Square protests.

Sohu.com issued a notice on its website (www.sohu.com) saying it was temporarily closing its chatroom, or online discussion forum, to "improve the system and services".

But an employee at Sohu said it was because the company feared online users might use the chatroom to post anti-government messages. "It was closed because of June 4th," the employee said.

Hundreds of people were killed when People's Liberation Army soldiers used machine-guns and tanks to storm central Beijing on June 3th-4th, 1989, crushing seven weeks of pro-democracy demonstrations centred on Tiananmen Square.

Chinese police require locally-based websites to monitor and remove sensitive content from their chatrooms.

The Sohu chatroom has more than 10 cyber-rooms where people can click into a communal forum and join electronic conversations.

But messages criticising the army crackdown on June 4th and a poem mourning those killed were posted on another popular Chinese chatroom at www.netease.com.

"How can we not talk about turmoil, rebellion? A disturbance that caused hundreds of thousands of soldiers to come out using tanks and guns to clear the road," wrote one participant who signed off "Don't forget June 4th".

The Tiananmen Square demonstrations are officially branded a counter-revolution. Authorities have defended the crackdown as necessary to save China from political and economic chaos.

"A stormy turmoil destroyed the youthful seedlings. . .let me write a mourning poem," wrote another signed "Yi Shuihan" - the name of a poem about a warrior who failed to assassinate the emperor.

In the run up to the anniversary of June 4th, telecommunications authorities in the eastern boomtown of Shanghai ordered local websites to seek official permission before posting political content.

The order also called on information vendors, including paging system operators, to clamp down on information that was seen as pornographic, superstitious or harmful to state security.

It has also restricted distribution of the Hong Kong-based Phoenix and Beijing has taken international French, German, Italian and Spanish channels off the air until June 24th "for maintenance".