BEIJING — After 80 days held without charge, China’s most voluble government critic, the artist Ai Weiwei, emerged from detention late on Wednesday night, thanked reporters for their concern and then did something almost unimaginable — he refused to say anything more.

“I can’t talk about the case,” he said Thursday afternoon, speaking outside his home, his generous girth visibly diminished by the months in custody. “I can’t say anything.” He then politely asked the nearly two dozen journalists to leave him in peace.

Although Mr. Ai was released on a form of bail that imposes a yearlong restriction on his movement and prohibits him from interfering with what the authorities described as an investigation into tax evasion, Chinese law does not explicitly prevent him from speaking about his ordeal — or any other matter.

But Mr. Ai, 54, who in recent years had become a persistent and freewheeling critic of Communist Party rule, has most certainly been instructed that his freedom depends in part on his ability to censor himself. Until April 3, when the police whisked him away from Beijing International Airport as he sought to board a plane for Hong Kong, Mr. Ai was an avid user of Twitter and a readily accessible source for foreign journalists seeking a barbed anti-establishment quote. His artwork, showcased in New York, London and Berlin, also provided searing critiques of government neglect and malfeasance.