China has closed down access to several of the world's most popular websites in an apparent attempt to censor international coverage of the violence that is unfolding in Lhasa, the Tibetan capital.

YouTube, the video-sharing website which has become a home to amateur footage of news events, has been blocked to Chinese users since Saturday, and there are also reports that the news pages of Yahoo!, the internet portal, have been made inaccessible.

In addition, the entire Guardian website has been closed down as of today, and other sites - including Times Online - have had access to their coverage of recent events in Tibet severely restricted.

Popular sites which assimilate news from different sources - such as Google News - have been subject to what is known as 'keyword filtering', where a Chinese internet user attempting to load a page which contains words such as 'Tibet' or 'Dalai Lama' will see the site stall.

Times Online has also learned that the editors of some of the most popular 'forum' - or bulletin board - sites in China have been directly contacted by government officials and told not to publish any content relating to the recent protests.

Flickr, the photo-sharing website, Wikipedia, and the LA Times, the US newspaper, are among the other sites to which access has been cut off.

"There's definitely been a ramping up of keyword filtering in recent days, particularly for words like Tibet and protest," said Jeremy Goldkorn, the editor of danwei.org, a site which translates news from various Chinese sources into English. "The whole internet has also slowed down, which is almost certainly connected with authorities' attempts to censor content."

The websites of most British newspapers are for the most part accessible, but since Friday, for instance, all articles by the Times Beijing correspondent, Jane Macartney, have been blocked to readers in China.

One comment on danwei.org today read: "I'm in the south of China, and many news sites containing Tibet-related articles are blocked with connection reset errors. This includes the entire Guardian website, as well as all news links from Yahoo."

Another read: "It's about midnight of the 16/17th, and Yahoo's homepage is blocked. They have headlines on the protests/riots up, so I'm not surprised."

China now has more than 210 million internet users - more than in the US, according to the government-backed China Network Information Centre - and authorities are notoriously strict about the sites which they are able to access.

YouTube has been blocked in the past, and the so-called Great Firewall of China prevents discussion of and searches for many sensitive topics, such as the Tiananmen Square protests.

Censorship is made easier by the fact that the country has relatively few internet service providers (ISPs) - the gateways through which all Western content must pass before it is seen from within China - meaning that software which runs 'keyword' checks on sites can readily be installed.

"In many ways, the technical solution - filtering software etc - is enough for the authorities here," Mr Goldkorn said. "You have to remember that the Chinese education system paints a very different picture of Tibet to that which is understood in the West, and it's likely many Chinese are simply not curious enough to try to make the effort to search out an alternative view of events."

Reporters Without Borders, the group which campaigns for press freedom, said today: "Yet again the Chinese government is trampling on the promises it made linked to the Olympics and has preparing the ground to crackdown on the Tibetan revolt in the absence of witnesses."

A spokesman for the Chinese Embassy in London was not available for comment.

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