Campaigners on Tuesday welcomed China's release of five feminist activists held for more than a month, saying the surprise move after an international outcry showed Beijing does sometimes respond to outside pressure.

The five, all aged 32 or younger, were taken into custody shortly before International Women's Day last month as they were preparing to hand out leaflets about sexual harassment on public transport.

The European Union, US Secretary of State John Kerry and his predecessor Hillary Clinton had all issued calls for their freedom, while Beijing described it as an internal issue.

Their lawyers said all five were released on bail on Monday, the deadline for prosecutors to formally charge them.

But authorities said Tuesday an anti-discrimination group which had called for their release, Yirenping, was suspected of breaking the law and would be punished.

China's ruling Communist Party does not tolerate organised opposition and often clamps down on small activist groups, with controls tightening since President Xi Jinping came to power in 2012.

Nonetheless the detention of the women -- Wei Tingting, Wang Man Zheng Churan, Wu Rongrong and Li Tingting -- was seen as unusually harsh given the small scale of their stunts, and the previous positive coverage they received in state-run media.

If Chinese activists are charged, prosecution and a guilty verdict normally follow but the women's release showed that Beijing had bowed to the "unprecedented global response" to their case, said Maya Wang, China researcher for Human Rights Watch.

Xi is expected to co-host a women's summit at the UN in September, and rights groups had called for a boycott of the event unless the five activists were released -- a potential embarrassment for Beijing, which is trying to build an image as a "responsible stakeholder" on the global stage.

"The optics of this arrest were obviously pretty terrible," said Joshua Rosenzweig, a human rights researcher based in Hong Kong.

The EU delegation to China said in a statement it noted the release "with relief".

- 'Ruled by law' -

The five were held in the cities of Beijing, Guangzhou and Hangzhou during a meeting of China's rubber-stamp parliament in the capital.

For such meetings security is stepped up nationwide and activists are often detained or warned not to travel.

The fact that the five were linked to actions in different cities may have raised concern among Communist authorities, even though they only highlighted issues such as domestic violence and the poor provision of women's toilets.

"Whenever you have cross-locality activism, that sets off alarm bells in the security apparatus," said Leta Hong Fincher, a Hong Kong-based writer on Chinese feminism.

Police originally told lawyers the activists were suspected of "picking quarrels and provoking trouble", a vague charge increasingly used under Xi to detain and jail protesters for holding small-scale demonstrations.

They later changed the accusation to "gathering a crowd to disrupt order in a public place", which carries the same maximum punishment of five years in prison.

Their release comes with conditions which will effectively prevent them from carrying out further protests, lawyers said.

"They will be monitored for a long time, they need to report their movements to police... and can be called in for questioning at any time," Wang Qiushi, a lawyer for Wei Tingting, told AFP.

Several of the women were involved with the advocacy group Yirenping, which campaigns to end discrimination against women, the disabled, people with HIV/AIDS and other marginalised groups.

It has come under increasing pressure over the past year and its office was raided last month, with several of its members subsequently going into hiding.

Foreign ministry spokesman Hong Lei said Yirenping was "suspected of violating the law and will face punishment in accordance with the law".

He denied Beijing had bowed to foreign pressure over the five, saying China was a country "ruled by law".

In a statement Yirenping co-founder Lu Jun said the organisation was "impressed by advocacy for their release from inside China and outside China".

"What they've done has actually furthered legal protection of women's rights and strengthened the rule of law in China," Lu said.