China has begun imposing an information blackout ahead of the 20th anniversary of the Tiananmen Square massacre, blocking access to popular networking websites such as Twitter and BBC television reports inside China.
The measures came as the authorities tried to close all avenues of dissent ahead of Thursday's anniversary, placing prominent critics under house arrest and banning newspaper from making any mention of the pro-democracy protests.
The co-ordinated internet "takedown" occurred at 5pm local time (10am GMT) on Tuesday as a broad range of websites suddenly became unavailable to Chinese internet users. Among the blocked sites was the blogging portal MSNSpaces, the Hotmail email service, Yahoo's photo-sharing site Flickr.com and Microsoft's new search engine Bing.com.
However, in a sign of how it is becoming increasingly difficult for the Chinese to control the internet, Twitter users found alternative outlets in rival providers to evade the censors.
Foreign newspaper and television channels were also subject to censorship as the highly sensitive anniversary approached.
Viewers of the BBC's world channels in Beijing found their screens turning black whenever reports on the anniversary were being aired and four foreign television crews attempting to film in Tiananmen Square reported being stopped by police.
Print publications were also affected, with many subscribers to The Economist magazine receiving their weekly copies with the Tiananmen-related pages ripped out. Readers of the Financial Times and South China Morning post also reported missing pages.
Over the last 20 years China's ruling Communist Party has refused to apologise for the deaths of hundreds, and possibly thousands, of protesters who were shot or crushed by tanks on the night of June 3-4, 1989.
Ignoring calls at home and abroad to pardon jailed demonstrators and "reassess" the events at Tiananmen, party officials have unwaveringly maintained the line that the protests were "counter-revolutionary riots" that were suppressed for the good of the country.
Qin Gang, foreign ministry spokesman, said yesterday: "The party and the government long ago reached a conclusion about the political incident that took place at the end of the 1980s and related issues."
The tightening of censorship ahead of Thursday's anniversary drew criticism from civil rights groups, including the press freedom watchdog Reporters Without Borders which issued a report condemning the lack of progress on media freedoms in China.
Renee Xia, of the banned Chinese Human Rights Defenders group, said the party continued to invest huge resources in keeping the Tiananmen massacre out of the public consciousness.
"Why? Because the Chinese leaders know they have blood on their hands. They fear that if the truth comes to light, the government will be under pressure to bring those responsible for this crime to justice," she said.
The Foreign Correspondents Club of China said it "deplored" the attempts to block reporting of the Tiananmen anniversary which it claimed went against the spirit of the relaxed reporting regulations that were introduced in the run-up to last year's Olympic Games in Beijing.
It accused the authorities of preventing at least four television crews from entering Tiananmen Square, harassing a reporter who interviewed the mothers of the victims and interrogating students who had given interviews.
As well media restrictions, the authorities have also silenced several leading dissidents, including Bao Tong, former secretary to Zhao Ziyang, the Communist Party leader purged for sympathising with the protesters. He was ordered to leave Beijing by security agents.
In another incident former Tiananmen Square protester who wrote an open letter to China's leaders complaining that political prisoners were still being refused jobs, pension and medical benefits 20 years after the massacre was also arrested, a human rights group reported.
Wu Gaoxing, a former teacher in his late 60s who was jailed for two years for his role in the protests was picked up in the eastern city of eastern city of Taizhou on Saturday, according to the New York-based group Human Rights in China.
Commemorations to mark the anniversary are being planned around the world, including Hong Kong where a small group of students have they will stage a 64-hour hunger-strike in commemoration the hunger-striking students of 1989.
In London Amnesty International UK said it was asking members to hold candlelit vigils while three former survivors of the repression laid flowers of commemoration outside the Chinese Embassy.
By Peter Foster in Beijing
Published: 1:40PM BST 02 Jun 2009