The women were taken into police custody last week in the run-up to International Women's Day on Sunday.
Activists said that the detainees were planning to hold public campaigns against sexual harassment.
Correspondents say the police appear to be holding at least one of them for an unusually lengthy period, as she was taken into custody on Friday.
Police in China usually release or charge a detainee within three days of their arrest, and Li Tingting, also known as Li Maizi, was detained on 6 March.
Chinese activist Feng Yuan, speaking from New York where she was participating in a UN women's issues events, said lawyers for the five in custody had not been able to reach them since their arrest.
"We're worried that they're still detained," Ms Feng told AP news agency. "We don't understand how this has to do with public safety. And this goes against what the Communist Party and the government says they want to do to build a safer, crime-free society."
This year's International Women's Day coincided with China's top political meetings and observers say Chinese authorities often detain activists before the start of major political or international meetings.
Eight women's rights activists were taken to police stations on Friday and Saturday, and three were released after a few hours.
One of those released told the BBC that the police told her to warn people not to take part in planned events.
Among the activities which the activists had planned were a march in a Beijing park where participants would wear stickers advocating safe sex and action against sexual harassment; and gatherings in Beijing and Guangzhou calling for awareness of sexual harassment on buses.
She added that the five who are still in detention are either members, or founders, of women's rights and gay rights groups in Beijing, Hangzhou and Guangzhou.
The irony is that the activists were detained for trying to promote women's rights to mark International Women's Day.  One of the activists - who did not want to be identified - told the BBC the police detained her for two hours over the weekend.
She had planned to take part in a protest at Beijing's Olympic Park to raise awareness about sexual harassment.
The campaigners held similar activities to mark International Women's Day in the past and faced no troubles. But this year's event coincided with China's annual parliamentary session, during which security is tighter than normal.
The police routinely detain people organising protests on vague charges of disturbing the public order.
Women's rights are high on this year's agenda - with the legislative session expected to pass a landmark domestic violence law. But increasingly the authorities are cracking down on non-governmental organisations pursuing their own agendas.
China is currently holding what is informally known as the "two sessions" - the keenly watched annual meetings of its legislature, the National People's Congress, and its advisory group, the Chinese People's Political Consultative Conference.
The sessions began last Thursday and are due to end on Sunday.
Over the weekend, female representatives of the congress held a news conference on gender equality and women's rights issues in China.
Official celebrations of International Women's Day were also held last week.
Supporters of five women's rights activists detained in China say they are concerned for their welfare.