At the rear of a white van, one policeman sat on each side of Mr. Ai, China’s most famous artist and provocateur. They clutched his arms. Four more men sat in the front rows.

“Until that moment I still had spirit, because it didn’t look real,” Mr. Ai said. “It was more like a performance. Why was it so dramatic?”

On the morning of April 3, 2011, the policemen drove Mr. Ai, one of the most outspoken critics of the Communist Party, to a rural detention center from Beijing Capital International Airport, where Mr. Ai had planned to fly to Hong Kong and Taiwan on business. So began one ofthe most closely watched human rights dramas in China of the past year.

China’s treatment of social critics has been thrust back into the spotlight by the diplomatic sparring over Chen Guangcheng, the persecuted rights advocate who left here on May 19 for the United States. A blind, self-taught lawyer, Mr. Chen pulled off a daring nighttime escape from house arrest. Like that case, the tale of Mr. Ai’s 81 days of illegal detention, recalled during a series of conversations in recent months, reveals the ways in which the most stubborn dissidents joust with their tormentors and try to maintain resistance in the face of seemingly absolute power. No critic has so publicly taunted the Communist Party as Mr. Ai, even as security officers have employed a variety of tactics in a continuing campaign to cow him.

Despite warnings from the authorities, Mr. Ai, 54, uses Twitter daily and meets with diplomats, journalists, artists and liberal Chinese. This month, a Beijing court agreed to hear a lawsuit that Mr. Ai has filed against local tax officials for demanding that he pay $2.4 million in back taxes and penalties. Last month, Mr. Ai set up four Web cameras to broadcast his daily home life, his way of mocking the police surveillance that surrounds him. Officers ordered him to stop.

“His personality is, ‘The more you push me, the harder I’m going to push back,’ ” said Liu Xiaoyuan, a lawyer and friend who was also detained last year.

During the 81 days, interrogators told Mr. Ai that the authorities would prosecute him for subversion, Mr. Ai said. The three main interrogators worked in an economic crimes unit of the Beijing police, and their aim was to gather evidence to charge him with subversion, tax evasion, pornography and bigamy. (Mr. Ai has a 3-year-old son from an extramarital relationship.) They questioned him repeatedly on his use of the Internet, his foreign contacts, the content of his artwork, its enormous sales value and a nude photography project from 2010.

Mr. Ai’s eyes grew moist when he recalled how interrogators threatened him with a dozen years in prison. “That was very painful,” he said, “because they kept saying, ‘You will never see your mother again,’ or ‘You will never see your son again.’ ”

In two different centers, Mr. Ai was confined to a cramped room with guards watching him around the clock. The second site, a military compound, was harsher, he said: lights remained on 24 hours, a loud fan whirred and two men in green uniforms stared silently from less than three feet away. Mr. Ai got two to five hours of sleep each night. He stuck to a minute-by-minute schedule dictating when he would eat, go to the toilet and take a shower. Mr. Ai, known for his portly frame, lost 28 pounds.

But the authorities at the military center ensured that he saw a doctor four to seven times a day. He received medicine for his many ailments: diabetes, high blood pressure, a heart condition and a head injury from a police beating in 2009. Mr. Ai noticed the hard-boiled egg on his breakfast tray each day had a tiny hole; a guard told him the authorities were keeping samples of each meal in case he got sick or died.

Mr. Ai’s ordeal began the morning that police officers drove him from the airport into the countryside. He was marched into a building and pushed into a chair.

“Stand up,” someone said.

Mr. Ai stood up. A man whipped off his hood. “I saw this tall guy right in front of me,” he said. “This guy looked like he was from an early James Bond movie.”

Mr. Ai thought he was about to get beaten. Instead, the man emptied Mr. Ai’s pockets and took his belt. His right hand was handcuffed to an arm of his chair.

The first team of interrogators arrived much later, at 10 p.m. One typed on a laptop, the other asked questions. The main interrogator, Mr. Li, about 40, wore a pinstriped sports jacket with leather elbow patches. He said he had never heard of Mr. Ai until he did an Internet search.

Mr. Li questioned Mr. Ai for more than two hours while chain smoking. He asked Mr. Ai about Internet chatter urging Chinese to start a “Jasmine Revolution.” Mr. Ai was questioned about a sculpture to be displayed in New York that consisted of 12 bronze heads of the Chinese Zodiac’s animals. Mr. Li accused Mr. Ai of not deserving credit for the work, since the display was modeled after a fountain at the old Summer Palace in Beijing, and workers had done the casting for him.

He also said he was surprised one head could sell for a half-million renminbi, or $80,000.

“Very few people know why art sells so high,” Mr. Ai replied. “I don’t even know.”

Mr. Li asked Mr. Ai about his extramarital relationship with the mother of his son. The policeman threatened Mr. Ai with a bigamy charge. “Don’t try to insult me,” Mr. Ai said. “You wouldn’t call that a marriage.”

As the two argued, Mr. Li took another tack.

“Your real crime will be subversion of state power,” Mr. Li said, as Mr. Ai recalled. “You scold the government all the time, you talk to foreign press all the time. We have to teach you something. We have to announce you’re a liar, you have economic problems and you married twice. And you put pornography on the Internet.”

So it went for about two weeks. Guards brought in a mattress each night. He was interrogated almost daily. Mr. Li alternated with a short, plump man named Mr. Liu.

The investigators were “respectful,” Mr. Ai said. Eventually he sensed them getting bored. Mr. Liu talked about noodle-making. The guards played with their cellphones. “You feel like a bead falling into a gap somewhere and you are forgotten, totally cut off from your connections and whatever experiences you had before,” Mr. Ai said.

The transfer to the second detention center happened without warning. Once again, officers hooded Mr. Ai. The guards were 80 young soldiers from the People’s Armed Police, a paramilitary force. They put Mr. Ai in Room 1135. White padding was taped to the walls, as in an asylum. The compound housed prominent suspects, including billionaires.

The new interrogator was sterner. One day, he and Mr. Ai mused on why Mr. Ai had embraced political activism. Was it because Mr. Ai had lived in New York for 11 years? Or because he had suffered during the Cultural Revolution? No, other Chinese had gone through those experiences and not been radicalized. The two men then hit on the reason: the Internet. Before Mr. Ai began blogging in 2005, he had been a stranger to computers.

On May 15, Mr. Ai was ordered to shower and put on a white dress shirt to see his wife. Mr. Ai knew the visit was for propaganda purposes and did not want to go. Officers told him he could say only three things: that he was being treated well; that he was being investigated for economic crimes; and that his family should not talk to journalists. Mr. Ai and his wife, Lu Qing, met for 15 minutes in the Chaoyang District police headquarters. “I didn’t even want to look at her,” he said. “It was completely insulting.”

Back in detention, the interrogations dragged on. One morning, the officers said they were sending Mr. Ai to prison, and asked him whom he wanted to see one last time. Then they said he might be released if he could persuade Ms. Lu to sign a document stating he was in charge of Beijing Fake Cultural Development, the company registered under Ms. Lu’s name. The police were building a tax case against the company, and the document would give them leverage over Mr. Ai.

The police called Ms. Lu. “Just sign whatever they want you to sign,” Mr. Ai told her.

She signed. Then officers sat Mr. Ai down in front of a videocamera and made him promise certain things: Never get on the Internet again. Never talk to foreigners. And so on. Mr. Ai signed a document saying he had been notified he owed back taxes. Officers blindfolded him for the drive to the Chaoyang police station.

At the station, he met his wife and mother. Together they went home.