At least 50 Chinese human rights lawyers and activists have been detained or questioned in recent days in an "unprecedented" police swoop, rights groups said Saturday, with around 20 still feared to be held.

"More than 50 lawyers and activists were targeted by police in a nationwide crackdown," Amnesty International said in a statement.

"All the individuals missing since the crackdown began on Thursday 9 July are well-known for their work on human rights cases," it added.

The best known was Li Heping, who was taken away from his home on Friday after what the New York Times described as a police search and the seizure of his computers.

Li had defended blind dissident Chen Guangcheng after Chen infuriated authorities in Shandong province by exposing forced abortions and sterilisations under China's one-child-only policy.

The scale of the clampdown on the legal profession began to emerge when a friend of lawyers and staff at a single Beijing law firm known for its human rights casework said at least five had been detained in the last couple of days.

"(They) were taken away by police for some 'investigation'... and they were not accused yet," fellow lawyer and close associate Zhang Qingfang told AFP.

All worked at Beijing Fengrui, a legal practice most recently in the news for taking on the case of Zhang Miao, a Chinese journalist detained for nine months after helping a German magazine report on democracy protests in Hong Kong.

Zhang's brother and a family friend said state security had detained her lawyer Zhou Shifeng on Friday, the day of her release, covering his head as they hustled him out of a Beijing hotel.

Fears were also raised for Wang Yu, another prominent Chinese rights defender at the firm, who had texted friends on Thursday to say her Internet connection was severed, before adding that unknown individuals were attempting to break into her home.

A legal assistant of Zhou's and an accountant at the firm had also disappeared along with the two lawyers, Zhang said.

Amnesty said earlier at least seven people working at the firm were known to be in the hands of the authorities or had failed to make contact with relatives in recent days.

"Wang Yu disappeared the day before yesterday, and no one has any indication where she is now," William Nee, a Hong Kong-based researcher for Amnesty, told AFP.

- National crackdown -

A wide-ranging crackdown on dissent has been under way since President Xi Jinping took office two years ago, with scores of government critics detained and dozens jailed.

Lawyers in Beijing, Guangzhou and Shanghai were all targeted, Amnesty said, but it was often difficult to ascertain whether they had been detained, taken in for questioning or had gone into hiding.

"We've seen detentions or people being disappeared or having their freedom of (movement) curtailed in many cities all around China. Clearly it is a nationwide crackdown. The most recent estimate we have is 48 people," Nee said.

One Beijing-based lawyer, Zhang Kai, sent a message that simply read "police have come", according to Amnesty, with relatives left in the dark ever since.

Another apparent victim was Liu Xiaoyuan, who has defended controversial artist Ai Weiwei and jailed Uighur intellectual Ilham Tohti. He had been tailed by agents, Amnesty said, until his family lost contact with him.

Meanwhile in the southern province of Guangdong, lawyer Sui Muqing was arrested on Friday for "picking quarrels and provoking troubles", Amnesty said, citing his wife.

The vague charge is increasingly used for a variety of public order offences and against activists who fail to toe the line of the authorities.

- 'Planned in advance' -

The Chinese Communist Party has repeatedly imprisoned those who openly challenge its right to rule or have protested publicly.

In an unusual admission, its state-run media said in December that torture by police to extract confessions is "not rare".

The timing of the police action was not a coincidence, prominent civil rights lawyer Li Fangping told AFP. "It goes further (than one firm)," he said. "(It was) planned in advance by the government".

Authorities have cracked down on activists, increased censorship of the media and Internet, and rolled out tough measures to confront what it labels "terrorism" in the far-western region of Xinjiang.

Despite the administration's emphasis on the "rule of law", the justice system is heavily weighted in favour of the authorities and the Communist Party.