The lawyer, Yu Wensheng, 48, said the police abruptly, and inexplicably, sent him home late Friday night after questioning him about a letter he had written to senior Chinese leaders, in which he criticized the government’s crackdown on lawyers who take on politically delicate cases.

The police officers who burst into his home late Thursday night and led him away in handcuffs had said he would face criminally charges for “picking quarrels and stirring up trouble,” according to his wife, Xu Yan. That vague charge has increasingly been used by the Chinese authorities to silence government critics and other perceived troublemakers.

Mr. Yu’s release was a rare and unexpected bit of good news for Chinese human rights lawyers. About two dozen Chinese legal professionals remain incommunicado many weeks after a wave of detentions that singled out lawyers who defend dissidents, outspoken Christians, farmers fighting government land grabs and other politically vulnerable clients.

In an interview on Saturday, Mr. Yu said he was stunned that the authorities had set him free.

“The police had said, ‘We will 100 percent, 10,000 percent, criminally detain you,’ ” he said by telephone. He said he thought that a surge of public pressure by friends and rights advocates might have played a role in his release.

Mr. Yu spent three months in police custody last year, and he said he had been repeatedly tortured during questioning. He was not charged with any crime, but the police warned him to keep quiet and threatened to detain him again at any time, he said.

Although he was handcuffed during his 24 hours in police custody this week, Mr. Yu said, his interrogators were comparatively civil.