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.

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.