A whistleblowing Russian lawyer whose death in custody became a symbol of rights abuses and strained relations with the United States will go on posthumous trial in what relatives say is revenge by the Kremlin.

Sergei Magnitsky, who died while in pre-trial custody in 2009, is being prosecuted on Monday for defrauding the state in what will be the first time Russia has ever tried a dead person, a development Amnesty International says sets a "dangerous precedent".

Magnitsky had been jailed after accusing police and tax officials of multimillion dollar tax fraud.

His employer says the charges against him were a reprisal and he was murdered, and the Kremlin's own human rights council aired suspicions he was beaten to death.

The circumstances of his demise led the United States last year to bar entry to Russians accused of involvement in his case or in other rights abuses.

Critics say the trial, which will begin on Monday, more than three years after he died and despite pleas by relatives to drop the case, is an attempt by President Vladimir Putin's government to hit back at Washington and show the public Magnitsky was a crook not a hero.

"It's inhuman to try a dead man. If I take part in this circus, I become an accomplice to this," Magnitsky's mother Natalya said.

"I won't take part in the hearings."

International criticism

Russia took the highly unusual step of reopening the investigation against Magnitsky in 2011, as international criticism of Russia over his death mounted.

"First they killed him, now they are dancing on his grave," said a lawyer for Magnitsky's family, Nikolai Gorokhov.

After Magnitsky's lawyers boycotted pre-trial hearings, the court appointed a lawyer to defend him.

Contacted by the Reuters news agency, the court-appointed lawyer Nikolai Guerasimov declined to comment on the case.

Putin's spokesman Dmitry Peskov also declined to comment.

Magnitsky died at the age of 37 after he said he was denied medical care over 358 days in jail.

Putin said Magnitsky died of heart failure, but his former employer, London-based investment fund Hermitage Capital, says he was killed for testifying against officials he accused of a $230 million theft through fraudulent tax refunds.

Hermitage owner William Browder is being tried in absentia alongside his former employee.

"Something happened in that prison that no one wants to talk about," Zoya Svetova, an investigator for the independent prison watchdog, the Public Oversight Commission, that probed his death.

"Magnitsky became a symbol of the fight against corruption, and the goal of this trial is to show he is no symbol but just a criminal who didn't pay his taxes," she said.

"It is pure state propaganda because there is no point in trying a dead man."