“Thanks to a mysterious twist of fate, this has become my life’s mission,” he said last year, weeks before he was detained by police.

Instead of admitting that he had been misled by “anti-Communist Party forces” like many other students did in forced confessions, Pu chastised the authorities for opening fire at the crackdown.

“That was definitely not what they wanted to hear,” he said. “Many people wrote stuff [to please the party], but I wasn’t one of them.”

Not having a job upon graduation, Pu had to settle for working at a wholesale vegetable market in Beijing. He was eventually offered a teaching position at a college after his former teachers put in recommendations for him, and he qualified as a lawyer in 1995.

Pu said his experience in 1989 only made him hold on more firmly to his values, inspiring him to take up a career that would enable him to fight for the rights of others.

“I have always upheld my convictions and these convictions have formed the way I live and think,” he said. “Nothing can change that – not even the police and state security agents.”

Over the years, Pu has defended writers in defamation cases or whose books were banned.

In 2004, he defended the authors of An Investigative Report of Chinese Peasants, a bestseller on the plight of farmers, in a libel suit. He also represented liberal writers Zhang Yihe and Dai Huang after their books were banned from being published in mainland China.

When he defended Tan Zuoren, an activist jailed on subversion charges for commemorating and writing about the Tiananmen crackdown – Pu even demanded, unsuccessfully, for former premier Li Peng and Beijing mayor Chen Xitong to give evidence.

Pu has taken on numerous cases that many other lawyers have shied away from.

He has defended dissident artist Ai Weiwei, Tang Hui – a woman who was sent for “re-education through labour” for seeking justice for her raped daughter – and Tibetan environmentalist Karma Samdrup.

Neither is the outspoken rights lawyer afraid of taking on sensitive issues.

Pu has advocated for the abolition of the notorious “re-education through labour” system, helped party cadres who were tortured in corruption investigations, and has even publicly accused former security chief Zhou Yongkang of rights abuses before Zhou was officially investigated.

Pu’s accusation of Zhou led to the authorities closing all of Pu’s microblog accounts.

To many, Pu, 50, a charismatic, burly figure with a deep, sonorous voice, was a reassuring and dependable figure. While many rights lawyers got into trouble with the authorities, he managed to avoid persecution, confident in his tact and understanding of the security system. He shrugged off the constant police surveillance he was under as something he had got used to.

But despite his poise, he was not immune from persecution in the end.

In May 2014, after Pu attended a private event marking the 25th anniversary of the Tiananmen crackdown, he was detained by police on the initial charge of “picking quarrels and provoking trouble”.

The police later added three more charges, accusing him of “inciting ethnic hatred”, “inciting separatism” and “illegally obtaining personal information”, but in May this year, the prosecution dropped the last two charges.

Pu was incriminated over the content of seven microblog messages he had posted between July 2011 and May 2014.

His commentaries were mostly sarcastic criticisms of the Communist Party and the government’s handling of an ethnic conflict in Kunming, Yunnan province, last year as well as barbed comments about two officials.

In one message, Pu slammed the hardline policies of former Xinjiang party chief Wang Lequan after the deadly attack that was blamed on Uygur separatists.

“I can believe that the terror was created by Xinjiang pro-independence [forces] – but this is the outcome, not the cause,” Pu wrote on March 2, 2014. “Wang Lequan ... you’re most familiar with that place; tell me: Why? Who are they aiming at?”

The 19-month detention without trial of Pu, who suffers from diabetes, heart disease and high blood pressure, prompted rights groups to condemn his treatment.

During his trial on December 14, Pu admitted that his microblog writing style was “sharp, caustic and sometimes vulgar” and offered to apologise to those who were offended, but insisted he had not warranted the charges of “inciting ethnic hatred” or “provoking trouble”, his lawyers said earlier.

Although his lawyer said he had never confessed, Xinhua news agency said on Tuesday that the court had shown leniency because he “pro-actively confessed to his crimes and showed remorse” and claimed Pu said “he has experienced the progress of rule of law, the improvement of the law and progress of society” through his ordeal.

Weeks before he was taken away by police last year, Pu cited a saying by ancient philosopher Mengzi as his motto: “When you’re rich, you should not succumb to indulgence; when you’re poor, you should not compromise your principles.”

It looks like Pu is once more paying the price for standing up for his beliefs.