Ai, an international artist who himself was detained for 80 days before being released in June, also described the effect of detention on two people who had been held the same time he was but who are now free.
In a tweet posted Tuesday, he urged support for Wang Lihong and Ruan Yunfei, who are being held by the authorities, the BBC said.
"If you don't speak out for Wang Lihong and don't speak out for Ruan Yunfei you are not only a person who doesn't stand up for justice and fairness, you don't have any self-respect," Ai wrote.
Ruan, a writer and dissident from Chengdu, has been detained since February and faces possible charges of subversion.
Wang, a human rights activist held since March, faces trial for "creating a public disturbance," a crime that can carry a sentence of up to five years in prison, the BBC said.
In a previous tweet, Ai named four friends who were detained alongside him and who were released at the end of June.
"Because they were connected to me, these four people, Liu Zhenggang, Hu Mingfen, Wen Tao, Zhang Jingsong, were illegally detained and though innocent, underwent great mental and physical torment," Ai wrote.
He said he met Liu for the first time since his release and he spoke about his detention. "Then tears fell from this tough man. He had a heart attack during detention and almost died," Ai said.
The BBC said a friend of Ai told them Ai wants Internet users to pay more attention to these two cases as they are "ordinary people" who will otherwise be "easily forgotten."
Ai reportedly said he will resume his campaigning against the government if his friends are convicted of any crimes.
More than 96,000 people follow Ai's tweets although people in China can't view the Twitter Web site because access is blocked by the so-called Great Firewall. However, it is possible to get around the controls.
Ai was detained on charges of tax fraud but denied the charges. His family insists he was targeted for his political activism.
He was detained April 3 at the Beijing airport as he waited to board a flight for Hong Kong in what was widely viewed as an attempt to silence a prominent critic while authorities had time to decide on legal grounds for prosecuting him.
Immediately after his release at the end of June, he told the BBC that his freedom was curtailed by the Chinese authorities.
"I am already home, released on bail, I can't talk to media but I am well, thanks for all the media attention," Ai, 54, said in an interview with the BBC.
China's state-run Xinhua news agency reported Beijing police released Ai "on bail and for his good behavior under arrest, for confessing his crimes and also because of "a chronic disease he suffers from."
Police said Ai had repeatedly said he was willing to pay the taxes he had evaded, Xinhua reported.
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