Mr Xu was escorted from his home by police in Beijing, who produced a formal notice saying he was being taken in for questioning.

The police also confiscated a fax machine, a computer, books and documents.

Mr Xu was one of about 30 dissidents from around the country who were planning to join the protest action in the eastern Shandong province. The protest concerned the alleged harassment by police of activist Xie Wanjun, who was involved setting up the China Democracy Party - a group which is trying to become the first registered opposition party since communist rule.

The dissidents say that Mr Xie has been ordered to surrender his house and that his wife has been forced to quit her teaching job because of his links with the party.

Among those planning to join the protest was Sha Yuguang, who has also been detained in Beijing. A third dissident, Qin Yongmin, who is in the central city of Wuhan, was held briefly on Monday night and told by police not to go to Shandong.

The BBC Beijing correspondent says dissidents have been taking advantage recently of a relaxation in the country's political atmosphere to test the limits of the government's tolerance.

Intellectuals' group banned

The arrests come shortly after the Chinese authorities ordered the closure of the China Development Union, an independent association of moderate intellectuals in Beijing which aimed to lobby the government to promote political and other reforms.

The group's leader, Peng Ming, said the authorities had declared it illegal, and that police had confiscated documents and computers.

The forum, which claimed 4,000 members, had been organising regular discussions in Beijing about political reform and other sensitive issues.

China hosts rights conference

Last week, amid heavy security, China opened its first international conference on human rights, and defended the way it treats its own citizens.