A Russian court unexpectedly acquitted opposition leader and chess grandmaster Garry Kasparov on Friday of participating in an unauthorized rally.

Kasparov, an anti-Vladimir Putin activist and former world chess champion, was detained Aug. 17, as he was attending a rally in support of the feminist punk band Pussy Riot outside the Moscow court where three of its members were being sentenced to two years in prison. He was violently rounded up by several riot policemen and thrown into a police van along with dozens of other protesters.

Kasparov also was later questioned after a police officer accused him of biting his hand during the detention.

But the Khamovniki court, where Pussy Riot members had been sentenced, cleared him of charges regarding the rally.

"This is a historic day — the courts have stopped trusting the police," Kasparov said after leaving the court room, according to his website.

Russian courts are notorious for jailing and fining opposition leaders and Kremlin critics. The law was recently changed regarding public gatherings to dramatically raise the fine for taking part in an unauthorized one to 300,000 rubles (about $9,000).

Kasparov says he's now planning to file a complaint against his detention and a libel case against his accuser.

Kasparov, who is considered by some to have been the best player in chess history, retired from top-level professional play in 2005 to become a political activist. He is a vocal critic of Putin and the leader of the United Civil Front opposition group.