Chinese artist Ai Weiwei has made his strongest anti-government remarks since his release from detention in June.
Mr Ai posted a message on the Twitter website saying one of his friends, held alongside him during his 80-day detention, had nearly died in custody.
He also urged people to support two human rights activists still being held by the Chinese authorities.
Mr Ai was detained on charges of tax fraud but his family insist he was targeted for his political activism.
More than 96,000 people follow Mr Ai's tweets and he had been using it and other sites to campaign against abuses of power and corruption in China.
Although people in China cannot view the Twitter website because access is blocked by the so-called Great Firewall, it is possible to get around the controls.
Mr Ai resumed posting messages at the weekend. His initial comments were innocuous, referring to what he was eating: "Lunch 10 dumplings, bodyweight regains 3kg."
But his latest messages on Twitter seem to be a challenge to China's authorities; resuming his criticism of them, and testing the limits of the restrictions imposed on his freedoms, says the BBC's Damian Grammaticas in Beijing.
One tweet names four friends who were detained alongside him.
"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," Mr Ai wrote.
He went on to say: "Today I met Liu Zhenggang. For the first time he began to talk about his recent detention... Then tears fell from this tough man. He had a heart attack during detention and almost died."
In another tweet posted on Tuesday, he urged support for Wang Lihong and Ruan Yunfei, who are being held by the authorities.
"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," wrote Mr Ai.
Ruan Yunfei, a prominent writer and dissident from the city of Chengdu, has been detained since February and faces possible charges of subversion.
Wang Lihong, a well-known 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 two were detained by police around the same time as Mr Ai.
The Communist Party, fearing a Middle-Eastern style Jasmine Revolution in China, had begun rounding up many prominent activists, says our correspondent.
A friend of Mr Ai has said that he wants internet users to pay more attention to these two cases as they are "ordinary people" who will otherwise be "easily forgotten".
Mr Ai has apparently said that if the two are found guilty he will resume his campaigning.