Lawyer Wang Qiushi, who represents activist Wei Tingting, said police told lawyers representing the women that they have been accused of creating a disturbance. Wang said it wasn't clear when the women were formally detained, which is a legal step before being tried in court, but police first informed him of the development on Monday.
He said if convicted of creating a disturbance, the women could be sentenced to up to three years in prison.
Wang said he had not yet been allowed to meet with his client and had not seen any written notice of her detention. Beijing police did not respond to a request seeking comment on the cases.
Police detained at least 10 women last weekend before they could put up posters in conjunction with International Women's Day in subway stations and other public transportation facilities in Beijing, Guangzhou and Hangzhou, said activist Feng Yuan. Police are still holding and have formally detained Wei, Li Tingting, Wang Man, Zheng Churan and Wu Rongrong, the founder of the Hangzhou-based group Women Center.
Lawyer Wang said the protest "would have helped society."
The case has fed fears that Chinese authorities are clamping down on public speech and dissent, with groups such as ones running community libraries recently singled out for official harassment.
The detentions also drew condemnation from Samantha Power, the US ambassador to the UN, who wrote on her Twitter account Thursday that the five should be released and that such actions restrict non-governmental organizations "fighting for universal rights."
The European Union also called for their release Thursday, saying in a statement that the action against the activists "violates their right to demonstrate peacefully.