The Chinese government has cracked down on international media coverage of the unrest in Tibet, blocking websites and censoring the local feeds of broadcasters including BBC World and CNN.

China's internet clampdown came over the weekend, following the outbreak of widespread unrest and violence in Tibet last week, and has hit websites including Yahoo, YouTube and Guardian.co.uk.

According to three separate reports posted on Danwei - a website specialising in the Chinese media and advertising industry - the Guardian website has been blocked, with Chinese users receiving a "connection reset" error. YouTube and Yahoo have also been affected, Danwei has reported.

One Chinese technology blogger told MediaGuardian.co.uk that the Chinese authorities' web censorship filtering system blocked access to particular stories.

"I am sure it is not a complete block, as the internet censorship system in China is based on a keyword filtering mechanism," he said.

He said that web users in China would have problems accessing almost all news websites reporting on Tibet if the stories contained keywords such as "Tibet", "violence", "riot" and "Lhasa".

Reports have emerged of access issues on a wide range of foreign sites.

"Different users will see different results as the filtering system doesn't always detect each web page, so users may be seeing different things depending on their location, the time and the frequency they access sensitive content," the blogger said.

A spokeswoman for BBC World said that its news coverage had been "periodically blacked out" when coverage relating to Tibet was aired.

A CNN spokeswoman also confirmed that its coverage of China had been the subject of censorship.

A blog posted by Jaime FlorCruz, the CNN Beijing bureau chief, indicates that CNN has been affected since Friday.

FlorCruz's post added that a local media blackout has meant that only a Chinese blog and a local version of Twitter that "collected and disseminated reports from citizen reporters" in Tibet were the only online news sources accessible within the country.

The BBC and CNN are only broadcast within international hotels and diplomatic compounds in China.

A spokeswoman for BBC news online pointed out that access to the corporation's website has already been restricted for quite some time within China.

"We understand there are reports of users being unable to access

YouTube within the People's Republic of China," said a spokesman for YouTube.

"We are looking into the matter, and working to ensure that the service is restored as soon as possible."

· To contact the MediaGuardian newsdesk email editor@mediatheguardian.com or phone 020 7239 9857. For all other inquiries please call the main Guardian switchboard on 020 7278 2332.

· If you are writing a comment for publication, please mark clearly "for publication".