MOSCOW: President Dmitry Medvedev ordered a probe into the prison death of a lawyer arrested in a high-profile tax case, the Kremlin said Tuesday, following allegations of "mediaeval" prison treatment in Russia.

Medvedev ordered the prosecutor general and the justice minister to investigate the death of Sergei Magnitsky, a 37-year-old lawyer working for Hermitage Capital Management, his spokeswoman told Russian agencies.

Magnitsky, who represented Hermitage in a tax investigation closely watched by foreign investors in Russia, died in a prison hospital in Moscow on November 16, with the official cause of death named as heart failure.

His lawyer, Dmitry Kharitonov, told the Interfax news agency last week that he had filed a complaint calling for a criminal inquiry into conduct by the investigators in the case, the head of the prison and its medical staff.

He said there were "signs of negligence, leaving a person in danger and failure to show medical support."

Magnitsky's mother, Natalya Magnitskaya, told Echo of Moscow radio station on Monday that her son was "methodically destroyed" by the courts and prison system, and that he didn't receive parcels of medicines and warm clothes.

A spokeswoman for the general prosecutor's office confirmed that an investigation into the lawyer's death had been opened.

"On the order of the president, the general prosecutor's office is carrying out a check of the circumstances of the death of Sergei Magnitsky in a pre-trial detention centre," the spokeswoman told the RIA Novosti news agency.

The death of the young lawyer, who worked for Moscow law firm Firestone Duncan, highlighted concerns about Russia's overcrowded prison system and the treatment of white-collar prisoners.

"A man who, from a legal point of view, was innocent died in prison in the most mediaeval way," Russia's main business daily Vedomosti wrote in an editorial last week.

The head of Hermitage Capital Management, Bill Browder, told the BBC in an interview published Monday that the lawyer was held "hostage" while the authorities tried to force him to sign false confessions.

"They basically said to him 'if you sign the following false confessions then we'll give you medical treatment - otherwise we won't'," Browder, who has been banned from Russia and lives in London, told the BBC.

Browder said the case showed Russia had become a "criminal state."

On Monday, human rights activist Ella Pamfilova tackled Medvedev on the death of Magnitsky at a meeting of top human rights activists at the Kremlin.

Magnitsky had been arrested in November 2008 on charges of tax evasion amounting to 500 million rubles (17 million dollars) in the probe that also indicted in absentia Hermitage's founder, Browder, who is a US citizen.

Medvedev "was obliged to act in some way," said Dmitry Oreshkin, an independent political analyst, saying that Magnitsky was "driven to his death."

"Such things can't be left without investigation because that would mean corruption runs riot and the president isn't in control of the situation in the country," he said.

Oreshkin questioned whether appealing to the prosecutors would be successful, however.

"I think doing this will be very difficult when we are talking about such a huge sum of money. I'm not even talking about a human life here," he said, adding that those involved in the case "are able to escape investigation."

Maria Lipman, an expert at the Carnegie Centre, urged caution over Medvedev's response to Magnitsky's death, which she called a "tragedy."

"I think an answer to this can only be given after the investigation has been carried out," she said. "It's easy to give an order, but it's more difficult to make sure that the order be implemented properly."