BEIJING - CHINA'S official Xinhua News Agency says a 28-year-old unemployed man convicted of killing six Shanghai policemen has been executed.

Xinhua said Yang Jia was executed on Wednesday morning in Shanghai after the country's highest court upheld his death sentence.

Yang was 28 and unemployed when he forced his way into a Shanghai police station July 1 and stabbed six officers to death. He was convicted in all six killings.

China's Supreme People's Court informed Yang's family Tuesday that a review of the case was complete and that the death sentence would be upheld, said Beijing lawyer Liu Xiaoyuan.

'From what I understand, (the execution) could be as soon as tomorrow, but we don't know,' said Mr Liu, who did not represent Yang during his trial but serves as lawyer for Yang's mother in a separate case.

The court made its decision on Friday, Mr Liu said.

Popular support for Yang swelled after local media reported that he told police the attack was revenge for the torture he allegedly suffered previously while being questioned by police about a stolen bicycle.

Yang tried to sue the police for psychological damage, but the claim was rejected.

During his trial, crowds of Yang supporters demonstrated outside the court and a group of prominent outspoken artists and intellectuals signed an online petition demanding the government investigate Yang's claims of abuse.

Even state-run media have asked what could have made a supposedly mild-mannered 28-year-old jobless man snap.

Yang lost his final court appeal in October, but all death sentences are subject to review by the country's highest court - a policy that began last year in an attempt to reduce China's number of executions, the highest in the world.

Yang's father said he and other family members were discussing the verdict but declined to comment further.

Zhou Shuguang, a popular blogger who signed the online petition, said he hoped that the execution would be stayed because doubts about the case remained.

'I hope they will not wrongly kill a man,' said Mr Zhou, who lives in central Hunan province and uses the online name 'Zuola'. 'They should postpone the execution to address all the unanswered questions about the case. That is the only way that the common people can believe that the law is fair.' Mr Zhou and other observers have also expressed outrage over the detention of Yang's mother, Wang Jinmei, shortly after her son's arrest.

Wang was held for four months in a police-run mental hospital before being taken Sunday to visit her son in prison and then escorted to her home in Beijing, Mr Liu said.

'She's wondering why the court didn't tell her earlier of their decision so that she would have known yesterday that it was probably the last time she would see her son,' Mr Liu said.

Yang's lawyer Zhai Jian said he was no longer involved in the case and was unaware of the supreme court ruling. -- AP