Pu had been in criminal detention since May 6 after he attended a small, private meeting on May 3 commemorating the 25th anniversary of the crackdown on the Tiananmen pro-democracy movement. He and four other participants were all detained on the charge of "picking quarrels and provoking trouble". All but Pu were released on bail last week.

Pu's lawyers, Zhang Sizhi and Li Jin, declined to comment on his case last night, saying they had not yet seen any official documents. Li said Pu's family had not been notified as of last night.

Li said the charge of "picking quarrels and provoking trouble" carried a maximum penalty of five years in jail, while the charge of "illegally obtaining personal information" carried a maximum jail term of three years.

Zhang, 87, already expressed fears this week that Pu might be indicted on several criminal charges and faced a heavy jail term. "The development of the situation is unfavourable towards [Pu], the heavy-handedness of the investigators ... is beyond my expectation," Zhang wrote in an essay.

"His case is not an ordinary case," he said.

Zhang also wrote that Pu, a diabetes sufferer, was interrogated nearly every day, sometimes for as long as 10 hours, and his legs had become swollen.

Last month, police took away Pu's aide and niece, Qu Zhenhong, also on suspicion of "illegally obtaining personal information". Also taken into custody last month were a Chinese assistant at Japan's newspaper and a Beijing-based NGO worker and former journalist. Both were Pu's associates and have been released on bail .

Legal scholar Teng Biao said the arrest of Pu was part of the authorities' wider crackdown on rights lawyers and activists.

"They are sending a message that even a well-known and non-radical lawyer can get arrested, let alone others," Teng said. "This is only a part of their targeting of civil society and activists."

Teng noted that four lawyers had been placed in criminal detention in the past few weeks and two legal professionals had been jailed this year.