FILE PHOTO - Zhang Zhiru, a prominent Shenzhen-based activist, holds a book before an interview in Guangzhou, Guangdong province, January 20, 2015. REUTERS/ Alexandra Harney

SHANGHAI (Reuters) - At least three prominent labour activists in China have been formally arrested on public order charges, a Hong Kong-based labour rights organisation said, the latest step by Chinese authorities in a rolling crackdown on civil society.

The three - Wu Guijun, Zhang Zhiru and He Yuancheng - have been in detention in the southern city of Shenzhen, near Hong Kong, for more than a month, the China Labour Bulletin said.

They learned on Tuesday and Wednesday that they were being formally charged with “gathering a crowd to disturb public order”, it said.

“The authorities are clearly nervous about the ability of labour activists to organise workers who are already struggling to make a living and increasingly angry at the gross social inequality they see around them every day,” CLB said in a statement.

It said it was unclear what incident the charges referred to because the authorities had told Wu and Zhang to decline the services of the lawyers hired by their families. It also said family members had been harassed and told not to talk to the media about the case.

Shenzhen police did not have an immediate comment about the arrests when contacted by phone.

Two other activists were detained on Jan. 20 along with Zhang, He and Wu, CLB said. “Given that they have yet to reappear in public, it is assumed that they have been arrested and charged as well,” it said.

Labour activists in China have long been under pressure from the ruling Communist Party, which is deeply suspicious of social organising that occurs outside of its control, but authorities have sharply intensified their efforts in recent years.