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France and Germany have called for the immediate release of a Chinese artist and dissident, detained in China.

Ai Weiwei has not been seen since he was stopped passing through security checks at Beijing airport for a flight to Hong Kong on Sunday.

German Foreign Minister Guido Westerwelle called on China for an "urgent explanation" of his fate.

A spokesman for the French foreign ministry said the country was "very concerned" about his disappearance.

"We hope he will be released as soon as possible," Bernard Valero said, adding that the French government was following events "very closely".

The authorities have not commented on the detention of the internationally acclaimed artist, who has now been missing for more than 24 hours.

Vocal critic

His wife told Agence France Press on Monday that police in Beijing had refused to disclose why they detained the artist.

Mr Westerwelle, who has just returned from a visit to Beijing and Tokyo, said he had impressed on Chinese leaders the importance of freedom of opinion and human rights during his trip.

"I call for an urgent explanation from the Chinese government and expect Ai Weiwei to be released without delay," he said.

Human rights group Amnesty International said Mr Ai's arrest showed that "China's time for open dissent has come to an end".

It called his detention a "troubling development" symptomatic of a "widening crackdown on dissent" across the country.

The artist, who has become one of China's most outspoken critics, complaining about a lack of human rights, was detained on Sunday morning while travelling with an assistant, Jennifer Ng.

House arrest

The documents of both were checked thoroughly before Ms Ng was allowed to continue on her journey to Hong Kong.

She has told the ����ý that Mr Ai was taken away by border guards and she was told to board the flight alone.

Police officers later raided the artist's Beijing studio, confiscating dozens of items and questioning several people.

Mr Ai helped design the main stadium used in the Beijing Olympic Games in 2008, known as the Bird's Nest.

His work often has political connotations and he has also lent his vocal support to others who have tackled the authorities.

Last year, the 53-year-old was prevented from travelling abroad and, in a separate incident, briefly held under house arrest.

He has been under constant surveillance and, according to his assistant, the police had visited his compound three times recently, prompting him to suspect he would soon be detained again.