China is holding at least five women's rights activists in detention after they planned a demonstration ahead of International Women's Day, which falls on Sunday, rights lawyers said.

Lawyers for two of the activists told Reuters the five women, who are from three cities, were detained by police on March 6. They were organizing demonstrations in several cities over the weekend against sexual harassment on public buses.

One of the women detained, Li Tingting, known by her pseudonym Li Maizi, is best known for organizing a campaign for gender-neutral toilets. Another activist, Zheng Churan, was detained in the southern city of Guangzhou. Both are in their early 20s.

Three other campaigners were detained in the eastern coastal city of Hangzhou and in Beijing.

Calls to public security authorities in Beijing went unanswered. A police official in Guangzhou said authorities were preparing a legal notice about Zheng's case for her family, and could not release information about her situation publicly.

The lawyers said charges against the women may be brought on Monday. Police have not said why they were detained.

"It's not as simple as I initially thought," said Yan Xin, Li's lawyer, by phone. "This is happening across cities. It seems the situation may be more complex."

Another rights activist who is close to Li and Zheng told Reuters that many other campaigners had been given warnings by police ahead of International Women's Day on March 8.

Rights groups say China's government is taking an increasingly tough line against freedom of expression. The detentions have also coincided with China's annual parliamentary session.