Washington on Wednesday demanded the release of one of China's most celebrated human rights lawyers and other activists detained during a round-up ahead of the sensitive 25th anniversary of the 1989 Tiananmen Square crackdown.

The US is "deeply concerned" over the reports of Tuesday's arrest of Pu Zhiqiang, said State Department spokeswoman Jen Psaki.

He and other activists were said to have been detained after taking part in a meeting to mark the June 4 anniversary "of the violent suppression of demonstrations in and around Beijing's Tiananmen Square," Psaki told reporters.

"We call on Chinese authorities to release these individuals immediately, remove restrictions on their freedom of movement and guarantee them the protections and the freedoms to which they are entitled under China's international human rights commitment," she added.

A lawyer confirmed Pu's arrest to AFP, while online reports said other academics and rights activists had also been detained after a seminar in Beijing on Saturday to mark the brutal suppression of huge anti-government protests in Tiananmen Square in 1989.

Authorities routinely round up outspoken critics of the Communist Party in the weeks before key events.

Pu, who has represented dissident artist Ai Weiwei, is a prominent rights campaigner and was held along with democracy activist Hu Shigen, an overseas rights group said.