Concern is growing for five Chinese feminists detained on suspicion of “picking quarrels and creating a disturbance” after planning to highlight sexual harassment on International Women’s Day.

The detainees are known for their work on women’s and LGBT rights and include some who have launched eye-catching, imaginative stunts to gain public attention.

Li Tingting, better known as Li Maizi, took part in an “Occupy the Men’s Toilets” campaign to demonstrate the inadequacy of women’s facilities and has paraded through the streets in a wedding dress splashed with blood to increase awareness of domestic violence.

From the Facebook ‘Free Chinese Feminists’ page. Li Maizi is on left. Photograph: Facebook

Authorities in Beijing, Guangzhou and Hangzhou seized nine activists just before International Women’s Day on Sunday, but have since released four. The remaining five reportedly planned to distribute stickers with slogans such as “Police: go arrest those who committed sexual harassment”.

The fact they are criminally detained – not just informally held – indicates they could well be charged. Detentions and convictions of activists have increased sharply since Xi Jinping became China’s leader two years ago and the women were seized during annual political meetings in Beijing, which tends to be a sensitive period. But similar initiatives to mark previous International Women’s Days had not led to custody.

The detentions took place as premier Li Keqiang met female legislators, telling them: “Women hold up half the sky [a famous quote from Mao Zedong] and you should believe that your male counterparts ... will move forward hand-in-hand with you.”

Feng Yuan, a women’s rights activist, said: “We cannot understand why the authorities are so tough this time. What the activists want is exactly what state policy on women says: that women should be equal.”

“There are so many mixed messages … there is some progress. Now, with these young activists detained, we see the progress to gender equality is so slow, our achievements are so little and the potential risk [to campaigners] is so huge.”

Last year, Li Tingting, 25, told the Guardian: “Gender discrimination is getting worse.” Another detainee, Zheng Churan, 25 – also known as “Datu” or “Big Rabbit” – complained that feminists were dismissed as man-haters.

The Chinese Human Rights Defenders network said the other women held are Wu Rongrong, 30, founder and executive director of the Weizhiming Women’s Centre in Guangzhou; Wei Tingting, 27, director of Beijing’s Ji’ande LGBT rights organisation; and Wang Man, who had worked on issues including gender equality in poverty eradication.

The Weizhiming Centre was also raided, said CHRD, adding: “The detentions and raids on NGO offices come at a time when the Chinese government is introducing strong measures, including a draft law to restrict the funding and the operations of foreign NGOs inside China, to further close off space for NGOs to function.”

Amnesty International said Wu had called a friend on the day of her arrest but all that could be heard were apparent cries of pain before the line was cut. The organisation’s China researcher, William Nee, added: “It is chilling that women calling on police to investigate sexual harassment end up as targets.

“The charges against all five women should be dropped and the women immediately and unconditionally released. The Chinese authorities should be working with these women to address sexual harassment, not persecuting them.”

Wang Qiushi, Wei Tingting’s lawyer, added: “The government has long called for the protection of women’s rights and promotion of gender equality, which is exactly what these feminists are doing. I think what they have done or were planning to do deserves to be praised rather than punished.”

Lü Pin, a friend of the detained feminists and women’s rights columnist, said: “If it’s as we have heard – they were planning to put up stickers on buses – it’s not at all in violation of the law, nor is it disturbing public order, let alone the fact that those plans did not happen.”

This year also marks the 20th anniversary of the United Nations’ fourth World Women’s Conference, held in China. It was regarded as a major step forward because it saw the adoption of the Beijing Declaration and Platform for Action, a blueprint for improving women’s status. There are concerns that it may be watered down.

Wang Zheng, professor of women’s studies at the University of Michigan, said 1,000 campaigners attending a meeting of the Commission on the Status of Women in New York had already joined the call for the five to be released.

She said she hoped the Chinese government would correct what she described as a “mistake” by public security authorities, to avoid damage to its international image as a supporter of gender equality.

“I would also hate to see the Chinese government’s plan to co-host the World Women’s Summit with the UN this September being set back by this episode,” she added.

Additional research by Luna Lin