By Matthew Davis
China has been aggressively censoring international media in an attempt to lock down information about the violent demonstrations in Tibet's capital, Lhasa.
China has stepped up its censorship of information in recent days
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.
Other broadcasters like CNN have also been affected - with transmission blocked, and reporters obstructed, amid tight controls on physical access to Tibet, which is off-limits to foreign reporters without a permit.
Writing on Tuesday for CNN's In The Field blog, correspondent John Vause said he and his crew were intercepted by Chinese police, some 300km from Ngawa county in Sichuan province, where Tibetan exile groups claim Chinese security forces have killed protesters.
"It had to happen - sooner or later they were going to find us and stop us. This morning they did. Chinese police, armed with those almost toy gun-looking small calibre machine guns pulled us over, asked for passports, and told us this was the end of the road," he writes.
Earlier, on the China Rises blog, journalist Tim Johnson noted: "We foreign reporters all take precautions. We have to switch vehicles often. Some of us swap out sim cards in our mobile phones, or just turn them off. That way, authorities cannot triangulate mobile phone signals and figure out our locations.
"None of us are doing anything illegal. It's just that it's very easy for officials in the hinterlands to stop us and ask endless questions, creating delays, or simply bar us from entering areas for unspecified security reasons."
Such posts are very often invisible to the estimated 137 million internet users inside the country, however.
China says Tibet was always part of its territory
Tibet enjoyed long periods of autonomy before 20th century
1950: China launched a military assault
Opposition to Chinese rule led to a bloody uprising in 1959
Tibet's spiritual leader the Dalai Lama fled to India
China operates a sophisticated keyword filtering system to censor internet content, which is capable of spotting homonyms and synonyms and even some kinds of rogue punctuation that internet users might use to sidestep the censorship.
Baidu, is the search engine most used by the Chinese - but this is heavily censored.
When the BBC tried searching on Google inside China for stories about "Tibet", the latest news stories about the protests were listed, but the stories themselves were not opening.
Searching on "Lhasa" brought up an error message saying "Connection was reset" - a familiar end to many searches for information about the protests. In recent days sites like Yahoo and Youtube have also been similarly affected.
However, while there are strong efforts to stem the flow of information, the proliferation of blogs, chatrooms and mobile phones has helped information about Tibetan protests to stream out.
Jeremy Goldkorn, editor of Danwei.org, which monitors China's media, said that new technology had forced the authorities to promptly acknowledge such events.
"They cannot lock down a disaster anymore," Mr Goldkorn told news agency AFP.
"Before the internet, it was possible in China to isolate an area because ordinary people did not have access to information, but that is not possible now."
Meanwhile, Chinese state media has been giving very different accounts of the violence than those found outside the country.
Chinese analysts have accused Western media of sympathising with the Tibetan people, giving prominence to critics of China's policies and reporting that hundreds of Tibetans have been injured by the security forces.
For the last few days, the state media has described the protesters as a small, politically-motivated group of criminals. State television has shown images of injured civilians in hospital, saying they were beaten or burned by the mob. The security forces have been painted as innocent victims.
At Premier Wen's news conference on Tuesday, he was asked four questions on Tibet by foreign journalists.
The state news agency Xinhua reported the government line about calm returning to Lhasa, while many other newspapers and web portals glossed over the issue, saying merely that the premier answered many questions, including on the Tibet issue, our correspondent says.
But analysts say the reporting of the protests is shaping attitudes within China, which were generally unsympathetic to Tibetans to start with.
The Chinese authorities have poured billions of dollars into building houses and big infrastructure projects in Tibet. Many Chinese view the region as a backward place which they are generously paying to develop.
They also view the Olympics as a focus of intense national pride. Attempts to undermine China's big moment are likely to spark anger.
The BBC's Jill McGivering says the differing perceptions of the violence are having a tangible effect on relations between China and the West.
In many ways they confirm the worst fears of both sides, she says.
The Western version of events fuels the belief that Beijing is still a repressive government, willing to use brutal methods to crush opposition.
In China, media reports match concerns that the world is ignorant, eager to demonise China and may even want to contain its rise, our correspondent says.