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Police in Hong Kong have arrested two activists for spray-painting slogans in support of the detained Chinese artist, Ai Weiwei.

The two men were later freed on bail.

Mr Ai - who in recent months became an outspoken critic of China's human rights record - has not been heard from since he, too, was arrested in Beijing last month.

Hong Kong's pro-democracy party said the arrest of the activists amounted to persecution.

The pro-democracy League of Social Democrats (LSD) described the arrests as "very wrong", the AFP news agency reported.

"The graffiti are a form of freedom of expression. The arrest was akin to helping the mainland Chinese government to crack down on dissidents," party vice-president Avery Ng told AFP.

Protesters have mounted a sporadic graffiti campaign in Hong Kong on behalf of Mr Ai.

The Chinese authorities say they are investigating him for economic crimes - as yet unspecified.

The artist was detained at Beijing airport while passing through security checks for a flight to Hong Kong.

Ai Weiwei's reputation is worldwide. He helped design the main stadium, known as the Bird's Nest, for the Beijing Olympic Games in 2008.