But the current shutdown appears to be one of the most wide-ranging yet aimed at stopping online discussion of the Tiananmen protests.

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 Interview: Tiananmen a 'tragedy'

 Tiananmen mothers demand justice

 The struggle behind Tiananmen

"Twenty years later, it is still impossible for the Chinese media to refer freely to the ruthless suppression of China's pro-democracy movement in June 1989," Reporters Without Borders said in a statement.

"The information blackout has been enforced so effectively for 20 years that most young Chinese are completely unaware of this major event."

Microsoft said its new Bing search engine, Live.com service as well as Hotmail had been blocked for Chinese users.

"We are reaching out to the government to understand this decision and find a way to move forward," Kevin Kutz, the company's director of public affairs said.

Yahoo, which owns the Flickr photo service, said it also had received no explanation from the Chinese government.

"We believe a broad restriction without a legal basis is inconsistent with the right to freedom of expression," said Jason Khoury, spokesman for Yahoo.

China's state media has stayed firmly silent about the anniversary [AFP/File photo]
The blockages form part of an increased security crackdown across China ahead of the highly-sensitive anniversary.

Foreign news broadcasts linked to the anniversary of the 1989 Tiananmen crackdown are also reported to have been were also cut.

Already-tight security around Tiananmen Square itself has been visibly stepped up with more police and plainclothes personnel deployed in and around the symbolic centre of political power in China.

The Foreign Correspondents' Club of China (FCCC) said it had received at least three reports of authorities blocking reporting at the square as well as claims of journalists or their sources being intimidated.

On Wednesday, police and paramilitary forces patrolled through crowds of tourists at the square.

'Scared'

"Business is poor today. You'd think most people are tourists but they aren't, they are plainclothes security," a trinket peddler surnamed Li, told Reuters, before police told her to stop talking to foreigners.

"They are scared there will be a big blow-up because of tomorrow, but I don't think anything will happen."

The crackdown on the Tiananmen protests was one of the bloodiest episodes in recent Chinese history but is virtually ignored in the tightly-controlled state media.

The official government line is that the protests were a "counter-revolutionary" conspiracy.

On the night of June 3-4, 1989, the Chinese government ordered the military to forcibly remove protesters from the square which had been occupied for almost five weeks.

The number of people killed that night and in the subsequent days remains unclear.

The government's official death toll stands at 241, including 36 students. But activists say thousands may have died.

Investigation

About 30 people are still serving prison sentences for their activities in the protests, according to Dui Hua, a US-based human rights organisation.

Others are in prison for continued activism after their initial release, and hundreds of other protest leaders are in permanent exile.

With the anniversary of the crackdown approaching, the US Congress on Wednesday called on China to launch a UN-backed investigation into the event and demanded the release of prisoners believed still to be in jail for taking part in the protests.

The House of Representatives backed a resolution that asks China "to invite full and independent investigations into the Tiananmen Square crackdown, assisted by the United Nations High Commissioner for Human Rights and the International Committee of the Red Cross".