China has blocked access to the video sharing site YouTube but the authorities insist that they are not scared of the internet.

Access to YouTube began to falter late on Monday and by Tuesday it had become impossible to reach the site.

The California-based company confirmed that the site had been blocked but was unable to provide a reason. It was working to restore access.

It may be no coincidence that the move has occurred around the anniversary of unrest in Tibet. The site was blocked last year from March 15 to 23, a day after a riot in Lhasa in which 22 people were killed after Tibetans protesting against Chinese rule rampaged through the streets of the capital city.

The release at the weekend of a video compiled by Tibetan exiles showing violent scenes shot by Chinese police of their crackdown on a riot in Lhasa in 1988 and a scene of armed police arresting protesters in March last year may have angered the authorities.

It is not the first time that YouTube has been blocked in China – or in other countries with governments nervous about the almost unfiltered content available to internet users. The site has previously been inaccessible in China for brief periods, sometimes with specific videos blocked.

Attempts to download certain videos on Monday produced that familiar message “Internet Explorer cannot display the webpage”. That usually means that the site is blocked.

A spokesman for the Chinese Foreign Ministry said: “Many people have a false impression that the Chinese government fears the internet. In fact it is just the opposite."

China had the world’s biggest number of internet users – about 300 million – and was home to 100 million blogs, which showed that the country's internet was open, he added. However, it also needed to be regulated by law to prevent the spread of harmful information and for national security.

YouTube is the latest of thousands of sites to be by what has become known as the Great Firewall of China – a phrase coined by the magazine Wired in 1997.

A crackdown that began in January has closed hundreds of Chinese sites, including a popular blog-hosting site and several sites popular with Tibetans. Analysts view it as another step in the Government's strategy to stifle dissent in a year of sensitive anniversaries, including the 20th of the bloody crackdown on the pro-democracy Tiananmen Square protests in 1989 and the 60th anniversary of the beginning of Communist Party rule.

Marc van der Chijs, a Dutch Internet entrepreneur who co-founded the Shanghai-based video-sharing website Tudou.com, offered another theory for the blockage in message on his website. "I suspect the real reason might be that YouTube just launched a Chinese version, which would make the site much more accessible for Chinese users. I don't like sites to be blocked; even not those of our competitors," he said.

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