Chinese police on Friday detained a prominent rights lawyer, his wife said, the latest of around 20 activists held on criminal charges ahead of the 25th anniversary of the Tiananmen Square crackdown.

China's ruling Communist Party forbids virtually all public discussion of the demonstrations crushed on June 4, 1989, when soldiers killed hundreds of pro-democracy protestors in Beijing.

Tang Jingling, an attorney who has represented uncompensated victims of land grabs and imprisoned rights defenders, was taken in on a charge of "causing a disturbance," his wife Wang Yanfang told AFP.

Police have criminally detained at least 19 other prominent liberal academics, lawyers and activists in the past month, according to the US-based group Human Rights in China.

China generally rounds up dissidents ahead of dates it considers sensitive, but rights groups say this year's detentions have been unusually widespread.

Those detained this month include Pu Zhiqiang, one of China's most celebrated human rights lawyers, who was held along with four others who participated in a private seminar discussing the Tiananmen protests.

The five were taken in on suspicion of "causing a disturbance", a vague charge that rights groups say is abused by police to round up dissenters.

At least four current or former journalists with connections to liberal activists have also been detained on criminal charges this month, according to Human Rights in China.

Japan's Nikkei newspaper said that a Chinese assistant at its Chongqing bureau in southwest China had been taken into custody on Tuesday by local authorities.

Police said the assistant was detained in connection with the investigation into Pu, the Nikkei said, after accompanying a journalist to an interview with the lawyer.

Also arrested this month were Wu Wei, a former reporter for Hong Kong's South China Morning Post, and Gao Yu, a veteran reporter previously jailed for her writing on the Tiananmen protests, who was accused of "leaking state secrets".

China's ruling party has pushed forward major economic reforms which have boosted growth since the Tiananmen crackdown, but has consistently suppressed activists it judges to pose a threat to its grip on power.

It has in previous years restricted the activities of the Tiananmen Mothers, a group who have campaigned for an apology from Chinese leaders after their children were killed in the crackdown.

At least four plain clothes security officers have been stationed outside the home of Zhang Xianling, whose 19-year-old son was shot dead by soldiers near the square.

"Their aim is to stop journalists and others from meeting me," she said. "It started from mid-April, earlier than previous years, and there are more people monitoring me."

Authorities have also prevented Ding Zilin, one of the more outspoken members of the group, from travelling to Beijing to pay her respects to the ashes of her son on the anniversary of his death, Zhang said.

"This is just going too far... it's an inhumane policy," she added. "I think they do it because they have something to be ashamed of."