Hundreds of backers of disgraced Chinese leader Bo Xilai have urged parliament in an online petition not to expose him to a potentially unfair trial over alleged abuse of power and other charges.

The letter on the leftist Red China website came as the top body of the National People's Congress opened a four-day meeting Tuesday during which it may decide to expel Bo as a legislator, which would strip him of his legal immunity.

The petition reflects support among left-leaning Chinese for Bo, who had championed a "red revival" before his downfall over corruption allegations and the murder of a British businessman, for which his wife has been convicted.

"The entire trial involving the Bo case has the problems of facts that are unclear, evidence that is neither reliable nor adequate and procedures that are not lawful," the letter said.

Bo, a divisive politician who once sat on the ruling Communist party's 25-member Politburo and ran the megacity of Chongqing, had been a candidate for promotion in China's once-in-a-decade power handover next month.

His downfall, triggered in February when his former police chief fled to a US consulate, exposed rifts among leaders and analysts say his fate became a bargaining chip in succession decisions in recent months.

The more than 500 signatories of the Red China petition, including former National Bureau of Statistics head Li Chengrui and a senior editor at the leading Shanghai Securities Daily, questioned if Bo's case was being handled fairly.

"Please share with the people evidence that Bo will be able to defend himself according to the law," said the letter on the Chinese-language website.

AFP was unable to confirm the participation of those named on the petition.

Last month state media announced that Bo would "face justice" for charges including abuse of power, taking bribes and improper sexual relations -- a much wider-ranging scope of allegations than many observers had expected.

It is unclear if any trial will be held before the leadership transition scheduled to take place at the party Congress that opens on November 8.

Bo's wife Gu Kailai was given a suspended death sentence in August for murdering a British associate and his former police chief, Wang Lijun, was handed 15 years the next month for covering up her crime.