The trial has begun in secret of the former police chief at the heart of China’s biggest political scandal in years, lawyers say.
Wang Lijun is charged with defection, abuse of power and bribe-taking.
He triggered events leading to the downfall of powerful politician Bo Xilai when he briefly fled to a US consulate in February.
Mr Bo’s wife has since been given a suspended death sentence for the murder of British businessman Neil Heywood.
The trial had been expected to start in Chengdu on Tuesday, but Mr Wang’s lawyer Wang Yuncai said the hearing had started earlier on Monday.
“The trial started at 08:30 this morning and finished at about noon,” Ms Wang, who is no relation to her client, was quoted as saying by The Daily Telegraph newspaper.
“The trial has two parts, private and public. Today is the private part because it involves state secrets – it was about the two charges of defection and bending the law for his own ends.”
Foreign journalists will not be given access to the public part of the trial, says the BBC’s John Sudworth outside the Chengdu court.
It is not clear how long Mr Wang’s trial will last but that of Mr Bo’s wife, Gu Kailai, lasted a day.
Key congress
The trial is taking place ahead of a key party leadership congress in China, expected in the coming weeks, and the Communist Party clearly wants the issue cleared up before that important political transition and leadership change, says our correspondent.
He adds that, had Mr Wang not gone to the Americans, Neil Heywood’s murder would probably never have been investigated, Gu Kailai would still be a free woman and her husband would most likely be about to win himself a seat on China’s most powerful political body – the standing committee of the Politburo.
Earlier Chinese state media reports said the evidence against Mr Wang was “concrete and abundant”.
The indictment against him said he knew that Gu was a murder suspect, but “consciously neglected his duty and bent the law for personal gain”, Xinhua news agency reported.
Mr Wang’s flight to the US consulate had proved an embarrassment for China and threw up issues involving diplomacy and state secrets, analysts say.
But most analysts expect him to be given a suspended death sentence, similar to the one meted out to Gu, reports say.
At a separate trial on 10 August, four senior police officers from Chongqing admitted to charges of covering up evidence linking Gu to the murder. A court official said they had been given terms of between five and 11 years in prison, AFP reported.
Tough reputation
Mr Bo, Mr Wang’s former boss in Chongqing, had been tipped for promotion to the top leadership ranks at the party congress before his downfall.
He has not been seen in public since the scandal erupted. He is said to be under investigation by the party’s disciplinary officials.
Mr Wang was seen as a loyal lieutenant of Mr Bo, but in early February the Chongqing city government said Mr Wang had been shifted to another job.
Four days later, he fled to the US consulate in nearby Chengdu, where many believe he sought asylum. He spent the night there but was persuaded to leave a day later. He gave himself up to police and has been in detention since then.
According to the UK Foreign Office, Mr Wang made allegations about Mr Heywood’s death while at the consulate.
Shortly afterwards, Mr Bo was sacked and his wife Gu was accused and later convicted of murdering Mr Heywood.
Mr Wang, 52 began his career in law enforcement in the Inner Mongolia Region in 1984 and moved to the southwestern city of Chongqing in 2008.
He had a reputation for being tough on organised crime and was once the subject of a TV drama called Iron-Blooded Police Spirits.
