Boris Berezovsky and Roman Abramovichhave appeared in a London courtroom for the first day of proceedings that pit the two Russian oligarchs against each other.
Berezovsky, who is exiled to Britain, is suing Abramovich, alleging breach of trust and breach of contract.
Berezovsky is seeking $5 billion in damages, claiming that Abramovich intimidated him into selling shares in the Russian oil firm Sibneft at a fraction of their value.
Abramovich denies the claims.
The case proceedings at the High Court are expected to last for more than two months.
Abramovich, the owner of Chelsea Football Club, sold Sibneft to Russia's state-owned gas monopoly Gazprom in 2005.
compiled from agency reports