China's Ai Weiwei speaks out over activists' detentions on Twitter

Mr Ai, who was banned from giving interviews when he was freed in June after 80 days in detention, said colleagues who were incarcerated "because of me" had suffered "great mental abuse and physical torture".

He revealed that Liu Zhenggang, a designer at his studio, had come close to death after suffering a heart attack during the time he spent in detention.

"Today I saw Liu Zhenggang, it was the first time he had spoken about this imprisonment," Mr Ai wrote in a tweet on Tuesday. "He raised his right hand and said, 'Sir, I want to drink water'. Then this strong man burst into tears ... He had a heart attack when was in prison and almost died."

Mr Ai declined to give an interview when contacted by The Daily Telegraph by phone on Tuesday evening, saying only that he believed he was keeping within his bail conditions despite the Tweets.

Asked to explain his decision to return to speaking out, Mr Ai – who has always drawn a distinction between 'posers' and 'doers' added only that if there were people suffering as a result of him, then he had no choice but to risk "putting myself in the same condition as them".

Mr Ai, whose Sunflower Seeds exhibition filled the Tate Modern last year, became the most high profile casualty of China's increasingly authoritarian stance following the Middle East uprisings, after which several prominent Chinese dissidents were detained.

In another tweet, Mr Ai said three other people who had worked with him – accountant Hu Mingfen; his assistant Wen Tao, and his cousin and driver Zhang Jinsong – had all been "illegally imprisoned". They and Liu had "innocently suffered great mental abuse and physical torture," he said.

Another Tweet urged his followers to speak out in support of two other dissidents, the renowned human rights activist Wang Lihong and Ran Yunfei, a writer.

Last Saturday, Mr Ai who has 96,000 followers on Twitter, released his first tweet since being taken into custody at Beijing's international airport on April 3 while trying to board a flight to Hong Kong.

Initially he confined himself to trivialities, including a post about the number of dumplings he had eaten for lunch, but now appears to be moving back towards a potential confrontation with the authorities.

Twitter is officially blocked in China but many web users still manage to access the site via virtual proxy networks (VPNs) that allow users to circumvent the so-called Great Firewall of China.

Chinese authorities have said the burly avant-garde artist, for several years an outspoken critic of the Communist Party, was detained for tax evasion, and barred him from leaving Beijing for a year after his release.

Rights groups have however said the outspoken 54-year-old was detained as part of a wider clampdown on activists launched in February.

The government said he was freed on June 22 because of his "good attitude" in admitting to the charges against him, his willingness to repay taxes he owes and on medical grounds. He has diabetes.