Russian opposition leader Alexei Navalny attends a hearing after being detained at the protest against corruption and demanding the resignation of Prime Minister Dmitry Medvedev, at the Tverskoi court in Moscow, Russia March 27, 2017. REUTERS/Tatyana Makeyeva

MOSCOW (Reuters) - One of Russia’s richest businessmen has filed a lawsuit against leading Kremlin critic Alexei Navalny, court documents released on Thursday showed, after alleging that Navalny had defamed him.

Navalny has emerged as a major irritant for the Kremlin after thousands of people across Russia attended anti-graft protests he organized last month. He says he plans to run against the Kremlin’s candidate in next year’s presidential election.

The suit against Navalny was filed in Moscow’s Lyublinsky district court by Alisher Usmanov, part-owner of British soccer club Arsenal whose business interests include iron ore producer Metalloinvest, and a major mobile phone operator.

The court documents did not include any details of the suit.

Navalny last month released a report alleging corruption involving Prime Minister Dmitry Medvedev. Medvedev dismissed the allegations as politically-motivated “nonsense.”

The same report included allegations relating to Usmanov. Usmanov, in interviews with Russian media, denied the allegations, accused Navalny of libel, and said he would take him to court.

Navalny said he stood by his allegations. “Excellent, let him do it,” Navalny said earlier this week in response to the prospect of Usmanov filing a defamation suit.