Chinese artist Ai Weiwei believed he was "close to death" during the more than 80 days he was held in a secret police detention centre, a source close to the artist told the BBC.
Beijing has banned him from speaking to the press since his release.
The source said Mr Ai was hooded when detained, kept under watch with guards inside his cell and subjected to treatment designed to break him.
Mr Ai was detained on charges of tax fraud by the Chinese authorities.
During his detention Ai Weiwei was, we were told, interrogated dozens of times. He was rarely asked about tax issues.
His interrogators focused instead on calls for a Jasmine revolution and claims Mr Ai was trying to "subvert the state", even though our source said the artist had no involvement in the calls and the interrogators had no proof.
The source told the BBC that during his 81 days in detention, Ai Weiwei was held first in one secret prison then moved to another, where he was kept in a windowless cell less than 4m by 4m (170 sq ft) in size.
The only daylight came from a small vent near the ceiling.
Ai Weiwei was never allowed outside, the source said. He exercised by spending five hours a day pacing back and forth across his cell, walking an estimated 9-12 miles (15-20km) a day.
There were two soldiers inside the cell 24 hours a day, standing to attention, staring at the artist. Our source said Ai Weiwei had to salute and ask them for permission to drink a glass of water, even to use the toilet in the cell.
The source has told the BBC that Mr Ai described his secret detention as "the toughest situation a human being can be in", and that every minute he felt "close to death".
China's most famous artist and one of the most vocal critics of its ruling Communist Party was picked up by police in April as authorities rounded-up activists, following online calls for a Middle Eastern-style Jasmine revolution in China.
Amid an international outcry, China's government said he was being investigated for tax evasion. But the source has told the BBC that when Mr Ai was released he was shocked to hear that the main accusation against him was that of economic crimes.
The state news agency, Xinhua, said in June that Mr Ai was released "because of his good attitude in confessing" to tax evasion and because he had agreed to pay back the money he owed.
But Mr Ai is contesting the tax claim, and the source said he was not guilty so never admitted any wrongdoing while he was being held.
Ai Weiwei was never formally arrested. His captors even told him there was no "legal basis" for holding him, but claimed they could do what they liked to him. "That way they completely break you," our source said.
The source said the police told Mr Ai "this is what happens when you criticise the government. You made China look bad, now we want to make you look bad".
And the source said that on his release the artist was also shocked to hear of the treatment of four of his associates, who he did not realise had been detained at the same time.
Wen Tao, a friend and former journalist, was apparently kept handcuffed for the entire 80 days he was held. Mr Ai's driver, Zhang Jinsong, spent 30 days in handcuffs, forced to wear them even in his sleep.
It is claimed Liu Zhenggang, the designer at Mr Ai's artistic company, was "taken like a hostage" and suffered a heart attack. One of the men was denied permission to wash during his detention.
Our source said stringent conditions had been imposed on Mr Ai upon his release, including a ban on talking to foreign journalists, a ban on meeting any foreigner, human rights lawyer or anyone involved in politics, and a ban on posting messages on the social networking site, Twitter.
But the source said Ai Weiwei remained committed "to speak out for people who have no voice of their own," because he believes: "if you don't speak out you let a lot of people down, you put them in danger."