Yang Jia, 28, an unemployed former supermarket clerk from Beijing, slipped into Zhabei police station, in a northern suburb of Shanghai, on July 1.

Once inside, he murdered four policemen with a knife on the first floor and one each on the ninth and eleventh floors before he was wrestled to the ground.

However, he attracted a wave of public support after being sentenced to death in a closed court. Thousands of Chinese lionised Yang for standing up to the security forces, which are increasingly reviled as the instruments by which the Communist Party stifles dissent and maintains a veneer of “social harmony”.

Sympathy for Yang grew after reports suggested that he was abused by the police on two separate occasions and may have been seeking revenge. Two years ago, he was allegedly badly injured by police in Shanxi province. Last year, he was arrested by police in Shanghai for riding a bicycle that they thought had been stolen, but which he had rented.

Yang’s defence alleged that the police had dragged him into a separate room and beaten him, an accusation that seems to be confirmed up by a tape recording of his interrogation, on which he can be heard being dragged away.

More than 4,000 people signed an open letter on the internet urging the authorities to spare Yang’s life. The letter was repeatedly erased from websites by state censors but remains in circulation. Celebrities including Ai Weiwei, the designer of Beijing’s Bird Nest stadium, and Mao Yushi, a Chinese economist, signed the petition.

Mr Mao said: “I think the chance to win the amnesty is very small, but we’ll do our best because it’s very important for China’s reform. I’m against the death sentence. I hope the death sentence will be gradually eliminated in China.”

The public outcry forced the authorities to allow Yang to appeal his sentence, even though he confessed to the crime in his first trial. At one appeal court hearing, hundreds of protesters, many of them middle-class, picketed the Shanghai Higher People’s Court and waved signs that read: “Love live the killer” and shouting “Down with the fascists!”.

Nevertheless the appeal was quashed in October and China’s Supreme Court ratified the death sentence. According to state media, Yang has now been executed. It did not say how he died but execution by firing squad is the most common method.

Several other aspects of his trial bolstered public opinion against the authorities. Initially, Yang was denied the opportunity to hire his own lawyer and allotted a public defender with close ties to Zhabei police station.

His mother disappeared, and was later discovered to have been forcibly detained for psychiatric treatment in Ankang Hospital, which is run by the public security bureau in Beijing. She was sent there on July 2, the day following the crime, and held for four months in a small cell without receiving any special treatment.

Finally, in October, the authorities denied a series of requests from Yang’s lawyers for information about his mother’s whereabouts and for the tape recordings of Yang’s alleged beating in Shanxi and of his detention in Shanghai.

After his execution, Ai Weiwei said: “Killing him in the name of justice is a humiliation to us all.”

Ran Yunfei, a blogger from Chengdu, wrote: “Yang’s murders were not justifiable, but the trial was an overt violation of the justice system, and Chinese have lost faith in the law.”

Other anonymous bloggers wished “Hero” Yang a “safe trip to heaven” and pledged to “always remember you — your name will be written in history”.