Once Russia's richest man, oil tycoon Mikhail Khodorkovsky is set to spend several more years behind bars after being found guilty of theft and money laundering charges.

Mikhail Khodorkovsky, former CEO of Russian oil group Yukos, on trial in Moscow (Image: Getty)

Khodorkovsky is nearing the end of an eight-year sentence for tax fraud in a highly politicised case seen as punishment for challenging the Kremlin's power.

Vladimir Putin, who was president during Khodorkovsky's first trial and is now prime minister, has shown no sign of softening his attitude towards the former oligarch.

Earlier this month, Putin compared Khodorkovsky to US fraudster Bernie Madoff. Putin said the oil baron was a convicted criminal that "should sit in jail" - a statement denounced by critics as interference in the trial.

Putin has not ruled out a return to the presidency in 2012 and critics suspect him of wanting to keep Khodorkovsky incarcerated until after that election.

Hundreds of Khodorkovsky supporters rallied outside the courthouse today, brandishing signs calling for "Freedom" and "Russia without Putin." Many were detained by police.

Khodorkovsky's conviction, on charges of stealing around US$27bn worth of oil produced by his Yukos company between 1998 and 2003 and laundering the proceeds, could keep him behind bars until at least 2017.

Reading the full verdict and announcing the sentence was expected to take several days, but it was clear from the opening pages of the verdict that the judge had found Khodorkovsky, 47, and his business partner Platon Lebedev guilty.

Prosecutors accused Khodorkovsky and Lebedev of stealing the oil from Yukos' own production units and then selling the oil abroad at higher prices.

The defence called the charges ridiculous, arguing that prosecutors did not understand the oil business, including the payment of transit fees and export duties.

Numerous witnesses, including current and former government officials, testified during the 20-month trial that the charges against Khodorkovsky and Lebedev were improbable, if not absurd.

One of Khodorkovsky's lawyers, Vadim Klyuvgant, has dismissed the charges as fabricated and claims that the guilty verdict is the result of official pressure.

In the courtroom, Judge Viktor Danilkin read the verdict in a low voice, drowned out at times by loud chants from outside.

In the wake of Khodorkovsky's arrest in October 2003 and the state takeover Yukos, the government has reasserted control over the energy sector.

Soon after Khodorkovsky's arrest, parties that he had funded were shut out of parliament or sidelined.

Russia's President Dmitry Medvedev has promised to strengthen the rule of law as part of his mission to modernise Russia and attract more foreign investment.

The outcome of the Khodorkovsky trial is seen as a test of whether Medvedev has any real intention - or real power - to deliver on his promises.

Medvedev urged officials on Friday to refrain from commenting on Khodorkovsky's case before the court rules - a statement seen by some as a tacit rebuke to Putin for his statement this month that the former tycoon deserves no leniency.