Two prominent legal advocates bound for an international law conference in London were blocked from leaving China on Tuesday on vague charges that their departure might endanger national security, the two men said.

Although the men, Mo Shaoping and He Weifang, said that while they were not given explicit reasons for why they were barred from their flight, they suspected that the government feared they would try to attend the awarding of the Nobel Peace Prize in Oslo next month to the Chinese dissident Liu Xiaobo.

Following last weekend’s deadly riots in its western region of Xinjiang, China’s central government has taken all the usual steps to block citizens from accessing foreign web services: aside from crippling Internet service in general, the authorities have blocked Twitter, removed unapproved references to the violence from search engines and has now apparently moved to bar its citizens from accessing Facebook from most parts of Mainland China just now. Two weeks ago, the government had already blocked just about every Google service, including communication tools like Gmail, Google Apps and Google Talk.

Dozens of prominent Chinese academics have signed a petition calling for the release of veteran political activist Liu Xiaobo.

They say his arrest shows that no one in China has the right to publicly express their opinions.

Mr Liu was formally arrested on Tuesday – more than six months after he was detained by the authorities.

He has been charged with inciting subversion by spreading rumours and defaming the government.

Mr Liu, who is a writer, has spent more than two decades pushing for political reforms in China.

Chinese officials have secretly executed a demonstrator who took part in a massive protest in 2004 against a hydro-electric dam in the south-western province of Sichuan, lawyers and family members said yesterday.

In a grim postscript to the summer of rural unrest that overtook China two years ago, Chen Tao was executed for “deliberately killing” a riot policeman during the demonstration, when 100,000 farmers staged a sit-in against the building of the 186-metre-high Pubugou dam on the Dadu river in Hanyuan county. The dam was set to flood thousands of people out of their homes and there were complaints that compensation was inadequate.

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