Wang Yu, a colleague of Mr. Zhou’s, disappeared from her home in Beijing in the early hours of Thursday, after sending friends a text message that her power and Internet connection had been shut off and, later, that people were trying to enter her home, Mr. Liu said. Last month, Xinhua, China’s main state-run news agency, condemned Ms. Wang in a commentary that called her a “hypocritical, bogus lawyer.”

Also taken from their homes by plainclothes officers on Friday morning were Li Shuyun, a probationary lawyer at the firm, and a paralegal, Liu Sixin, Mr. Liu said, citing Ms. Li’s sister and a phone call from the detained paralegal and the accounts of his relatives.

On Friday afternoon, the police also searched the Fengrui offices and took away at least three computers, Mr. Liu said. “Other people in the law firm have also been detained, but it’s hard to count, because some maybe are hiding and turned their phones off,” he added. “I’m also in danger and staying outside Beijing.”

The firm has dozens of lawyers, and its website features inspirational quotes from Nelson Mandela and the slogan “Equality, freedom, democracy, legality.”

The police have not confirmed that they are holding the four lawyers or the paralegal. But their supporters said they appeared to be the latest citizens detained for taking up politically contentious cases and causes that have riled the Communist Party. Since President Xi Jinping took power as party leader in November 2012, his government has enforced a sweeping crackdown on dissidents, rights advocates and lawyers who try to defend them.

The Fengrui Law Firm “provided a base to a number of human rights lawyers,” said Eva Pils, a law scholar at Kings College London who studies China’s rights defense lawyers. “There is a sense that to control this restive and growing group of human rights lawyers, they have to do something that goes beyond the level of repression that everybody has already gotten used to.”

Li Heping, who did not belong to the same firm as the others who disappeared, has acted as the lawyer for some of China’s most prominent dissidents and rights campaigners. One client was Chen Guangcheng, the blind legal advocate who escaped house arrest in his village in 2012, fled to the American Embassy in Beijing and eventually settled in the United States.