Rights groups say the current clampdown on dissent is the most sweeping in two decades in China, where a slowing economy has led to a surge in labour disputes, particularly in the southern manufacturing powerhouse of Guangdong.
Zeng Feiyang, director of the Panyu Migrant Workers Centre in the southern city of Guangzhou, was charged with "disturbing social order", said Cheng Zhunqiang, his lawyer.
Zeng is one of China's most prominent labour activists, many of whom have campaigned for the legal rights of workers, such as proper work contracts and social insurance contributions.
Two other activists, Meng Han and Zhu Xiaomei, have also been arrested on the same charge, said Yan Xin, Meng's lawyer, and Cheng. He Xiaobo was arrested on a charge of embezzlement, according to New York-based rights group China Labor Watch. The lawyers for Zhu and He could not be reached for comment.
Both Cheng and Yan told Reuters by telephone that prosecutors in Guangzhou told them of the arrests of Zeng and Meng on Friday, but did not give any reason for the charges.
Both lawyers said they had been unable to meet their clients since their detentions, in contravention of Chinese law.
Prosecutors in Guangzhou's Panyu district did not answer Reuters' repeated telephone calls to seek comment. The Guangdong government did not respond to a faxed query.
A formal arrest usually leads to a trial. Last month, police in Guangzhou detained seven labour activists, sparking criticism from rights groups. Two of them have since been released, Cheng said.
At the time, state media accused the seven detained labour activists of "inciting workers to go on strike", accepting foreign funding and "disturbing social order".
They also said the married Zeng had "at least eight long-term lovers", a charge that Zeng's supporters call a smear against him.
The number of strikes in China surged to a record 2,774 last year, or double the figure for 2014, Hong Kong-based advocacy group China Labour Bulletin said last week.