Russian tycoon Alisher Usmanov and opposition leader Aleksei Navalny have exchanged bitter, strongly worded video attacks as they rejected a Moscow court's proposed settlement in a defamation case.
Usmanov, an Uzbek-born billionaire with ties to the Kremlin, filed the lawsuit in April against Navalny and his Anticorruption Foundation.
The defamation claim stems from a March 2 report by Navalny's foundation that focused on Prime Minister Dmitry Medvedev and included allegations that Usmanov gave expensive property to a foundation linked to Medvedev at no cost.
Hours before a scheduled preliminary hearing in the case on May 17, Usmanov released a video on YouTube calling Navalny a "loser" and a "failed businessman."
Speaking in a manner that for some viewers evoked the jargon of Russian crime bosses, he addressed Navalny by the familiar form of 'you' and by the nickname Lyosha -- short for Aleksei.
WATCH: Alisher Usmanov Criticzes Aleksei Navalny (no English subtitles)
"Your attempts to slander me are like a puppy's barking at an elephant," said Usmanov, who pointed to Navalny's two politically charged financial-crimes convictions and called him a criminal.
He urged Navalny to apologize and made a sound suggesting he was spitting on Navalny.
After the parties rejected the Lyublino District Court's proposed settlement, Navalny fired back with a video, in which he called Usmanov "a beginning blogger," and "a bribe-taker, a bribe-giver, a crook, and a fraudulent man."
WATCH: Navalny Responds To Alisher Usmanov (no English subtitles)
Usmanov -- now one of Russia's richest men, with assets in mining, media, and other industries -- spent six years in a Soviet prison in the 1980s but his conviction was later overturned. He was exonerated in 2000 by Uzbekistan's Supreme Court, which ruled that the case against him had been fabricated and no crime was ever committed.
Usmanov's bearing and body language in the video drew mockery on social media, but Russian Foreign Ministry spokeswoman Maria Zakharova came to his defense on Facebook, praising him for what she called a "man's" response and saying his video will be studied "at universities by rhetoric and PR specialists."
Navalny's allegations against Medvedev helped galvanize nationwide anticorruption protests on March 26.
Navalny, who was jailed for 15 days over the protests, is calling for a fresh anticorruption demonstration on June 12.
He wants to run in a March 2018 election in which President Vladimir Putin is widely expected to seek and secure a new six-year term.
He announced in December that he would run for president in a March 2018 election in which President Vladimir Putin is widely expected to seek and secure a new six-year term.
Russian authorities have suggested that Navalny would be barred from the presidential ballot due to his financial-crimes convictions.
However, Navalny's backers say the rules are unclear and the anticorruption activist has pushed ahead with his campaign.
Election officials have still not stated clearly whether Navalny will be allowed to run for election.