The secret detention was "the toughest situation a human being can be in" and he felt "close to death" every minute, the source told the BBC.
Ai is banned from making statements to the press or online but Wednesday also made a Twitter posting and spoke with The New York Times to denounce the treatment of business colleagues who had been detained with him and to express support for two other incarcerated dissidents.
The artist who is also active in architecture, photography and film told the Times he felt compelled to speak out on his colleagues' behalf, even though it might cause further legal difficulties for him. He said his colleagues were in greater danger in prison than he had been because they were unknown to the general public and faced harsher treatment as a result.
He said on Twitter they faced "huge mental devastation and physical torture," and one almost died after suffering a heart attack.
"If I don't speak out for them, this is not possible, even though it may bring damage to my condition," he told the newspaper in English.
Ai was held by state security officials with a hood over his head for 81 days in a windowless, 13-by-13-foot cell.
He was interrogated repeatedly and watched over at all times by two soldiers who stood at attention inside his cell, the BBC said.
He had to ask their permission to do anything, including use the toilet or drink a glass of water, the BBC said.
He was arrested at Beijing Capital International Airport April 3 in a round-up of activists following growing online calls for a Chinese revolution similar to the so-called Jasmine Revolution in Tunisia that led to the ouster of longtime President Zine el-Abidine Ben Ali.
Chinese activists staged weekly pro-democracy protests in more than a dozen Chinese cities starting in late February. At least 35 activists and lawyers were arrested, with a number of them charged with "inciting subversion of state power," a charge frequently given against human-rights activists.
Ai was charged with tax evasion arising from his Shanghai art studio business. He was released nearly two months ago after Chinese authorities said he confessed and was a model prisoner. His tax-evasion case is pending.
During his imprisonment police rarely mentioned tax evasion and focused instead on the Jasmine Revolution calls that Ai asserted he had nothing to do with, the source told the BBC.
"This is what happens when you criticize the government," the source said police told Ai. "You made China look bad. Now we want to make you look bad."