HONG KONG — The Chinese police have formally arrested four human rights advocates in the past week on the charge of subverting state power, after detaining them for six months, according to one of their colleagues and rights groups.

The families of two lawyers, Zhou Shifeng and Wang Quanzhang, both of the Fengrui Law Firm in Beijing, received letters Tuesday that notified them of their relatives’ formal arrest, their colleague, Liu Xiaoyuan, said by telephone. Zhou is the director of the firm. Liu said Li Shuyun, an intern lawyer at the firm, was also arrested under the charge, as was Zhao Wei, an assistant to another rights lawyer. The letters were dated Friday.

The four rights defenders were part of what was until last year a flourishing group of legal experts who represented prominent Chinese clients, including the artist Ai Weiwei, the blind activist Chen Guangcheng, and the Uighur scholar Ilham Tohti, as well as ordinary people seeking justice through the Communist Party-controlled court system. In July, more than 200 of these experts were rounded up in a nationwide sweep and pilloried by the state-run media as swindlers. Many were detained at an undisclosed location in the port city of Tianjin.

The charge of subverting state power, which can carry a sentence of up to life in prison, is far more serious than several human rights advocates had expected in the four recent cases and suggests that the government believes that these people were seeking to undermine the state through their legal work. By way of comparison, the Nobel Peace laureate Liu Xiaobo was sentenced to 11 years in prison in 2009 on the charge of “inciting subversion of state power,’’ regarded by many as a lesser offense.

The crackdown on lawyers is part of a wide-ranging constriction of civil society under President Xi Jinping, who is intent on shoring up the Communist Party, which has ruled China since 1949.

New York Times