MOSCOW (AP) — Hearings have opened in the defamation case between Russian opposition leader Alexei Navalny and multi-billionaire Alisher Usmanov.

Usmanov, estimated to be worth $15 billion, has sued Navalny over his investigation into Prime Minister Dmitry Medvedev's alleged wealth. Navalny published legal documents showing that the oligarch, along with several other Russian billionaires, transferred lucrative assets to a charity foundation run by Medvedev's former classmate.

Earlier this month the reclusive Usmanov released a YouTube video dismissing Navalny as a "loser." A week later, Usmanov released another angry video, signing off with "I spit on you again."

The 63-year-old tycoon, whose business empire spans from mining to media, heads the International Fencing Federation and is a minority shareholder in the Arsenal football club in the English Premier League.