A Moscow court on Friday started a preliminary hearing on the case of Mikhail Khodorkovsky, former chief executive of Russia's embattled oil giant Yukos.

Charged with seven offenses including fraud and tax evasion, Khodorkovsky, Russia's richest man, will face up to 10 years in prison if convicted guilty.

Khodorkovsky was brought into Moscow's Meshchansky Court and the hearing was being held behind closed doors, Interfax reported.

A small group of Khodorkovsky's supporters gathered outside the court, demanding that the oil tycoon, who has been detained since October last year, be released immediately. Many police officers were on guard around the court building.

The case is expected to last for a long time and Friday's hearing may rule whether the court would approve the merging of the cases of Khodorkovsky and Platon Lebedev, a key Yukos shareholder who has also been jailed on similar charges. Lebedev is expected to appear in the same court later Friday.

Critics see the judicial probe into Yukos and the arrest of Khodorkovsky, who had reportedly supported the political opponents to Russian President Vladimir Putin, as a Kremlin-backed effort tostifle the oil magnate's financial and political clout.

However, Putin has repeatedly denied any political background behind the case, saying the probe is part of the anti-corruption campaign.