China has been aggressively censoring international media in an attempt to lock down information about the violent demonstrations in Tibet's capital, Lhasa.

The anti-Chinese protests are an extremely sensitive issue for Beijing, which is desperate to avoid bad publicity only months before the Olympic Games.

In recent days, TV broadcasts have been blacked out, websites blocked or censored by China's keyword filtering system and reporters on the ground prevented from reaching the region.

The degree of censorship appears to be fluctuating and uneven, however.

On Friday - said to have been the worst day of violence in Tibet since demonstrations in 1989 - the first few live interviews on BBC World with correspondent Daniel Griffiths were blocked from local transmissions.

But repeat broadcasts of these interviews were allowed to go ahead.

In one particular hour, correspondents talking about Tibet from London could be seen on screen, but when the story shifted to a BBC correspondent in India, TV sets in Beijing went black.

On Monday and Tuesday, such censorship appeared to be less frequent.

Yet one line in a piece on Chinese Premier Wen Jiabao's news conference, by the BBC's Quentin Somerville, appears to have irked the censors.

When our reporter said: "Overnight, China's deadline passed for protesters in the city to stand down - there's no immediate word on the fate of those involved in the protests", the transmission was momentarily blocked.

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