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.