Ai Weiwei, a dissident artist who was released last month after spending 81 days in a Beijing jail on tax evasion charges, spent his days of confinement in a tiny room with only a bed for furniture and two police officers who monitored his every movement, even standing next to him in the shower, his sister said Thursday.

For exercise, Ai paced in the small room, only six floor tiles long, but he estimates he walked the equivalent of 600 miles and lost about 30 pounds. As he paced, the two officers walked behind him, said his older sister, Gao Ge. The room had one covered window.

Ai, one of China’s leading contemporary artists, was among dozens of activists — human rights lawyers, bloggers and others — detained this year after anonymous Internet calls for a Middle East-style “jasmine revolution” against China’s ruling Communist Party. His arrest touched off an international firestorm of criticism, although Chinese officials denied that foreign pressure played a role in his release.

Since he was freed, the normally outspoken Ai has been prohibited from speaking publicly or giving interviews, his Twitter account has remained dormant and he requires permission to leave his Beijing home, including to visit his mother’s house. Gao’s account provided the first details of what happened to Ai during his detention, when he was allowed only one visit with his wife.

Gao spoke just before Ai’s wife, Lu Qing, and his attorneys appeared Thursday at the Beijing Taxation Bureau to challenge charges that Ai owes $1.8 million in back taxes and fines for a design company he is said to control, called Beijing Fake Cultural Development Ltd. Lu is the company’s legal representative.

The closed-door hearing, which lasted three hours, did not resolve the dispute. Afterward, Lu and Ai’s attorney, Xia Lin, said the process was unfair because they were not allowed access to documents that the government said had been seized during a raid of Ai’s studio.

“We could not really defend ourselves,” Xia said in an interview after the morning session. “I protested twice in the hearing for it not being open and for their not returning our documents.” He said the authorities showed only what they said were photocopies of the original documents, and he could not tell whether they were legitimate or had been doctored.

Ai’s sister, Gao, said her brother was innocent of tax evasion and was not involved with the company. She said he had signed a statement accepting responsibility and agreeing to bail only because he was worried that his wife would be targeted.

Gao said that during his time in jail, Ai was not tortured and was given food and allowed to take his medications regularly. But, she said, the conditions of his detention constituted psychological pressure.

“The room light was on 24 hours every day,” she said. “The only furniture in the room was a bed. Except for the bed, there was nothing else in the room, no chair, no desk. They didn’t offer Ai anything— no book, no newspaper, no TV, no radio, not even a piece of paper or a pen.”

Gao said the two guards watched him constantly, never speaking; the officers changed shifts every three hours.

“They stared at him without ever moving their eyes,” she said, adding that they stood close by even while he used the toilet. “And when he took a shower, they just stood right next to him, even though they were getting totally wet.

Can you imagine the feeling of having four eyes always on you, no matter what you do?” Gao said. “If you lie down and go to sleep, they just stand at the side of the bed and look at you without a blink of the eye. When he had a walk in the room, they also followed him. These measures were designed to destroy people’s minds,” she said.

Ai’s relatives and supporters said he has been in good health since he was released on bail June 22 and spends most of his days with his son. He is said to be delving back into his creative work, having been inspired by his confinement.

“Ai Weiwei is all fine now,” Lu said. “His health is good, and his mood is okay. It’s really been a unique experience for him.”

“As you know, as an artist, he always has a lot of ideas,” Lu said. “He is working on those ideas now. He stays at home, visits friends and receives visiting friends nowadays.”

On Wednesday, Berlin’s University of the Arts said that it had offered Ai a position as a guest lecturer and that he had accepted. But with his passport confiscated, it was unclear whether he could take up the position. Before his April 3 arrest, Ai had announced plans to set up a studio in Germany because of the restrictions on artistic freedom in China.

Gao said, “If the country is really not suitable for him to stay, we can’t exclude the possibility that our family will all leave the country. My only hope is that he won’t be thrown into prison again.”

Researcher Zhang Jie in Beijing contributed to this report.