The leader of the Russian opposition PARNAS party, Mikhail Kasyanov, has been reportedly attacked by unknown individuals in the southern city of Stavropol.
Andrei Zubov of PARNAS told the Dozhd TV channel that some 25-30 people attacked Kasyanov near a hotel on August 10.
The attackers hit Kasyanov several times and tore his shirt, Zubov said.
He added that the attackers were supporters of the pro-Kremlin National Liberation Movement (NOD), saying he had seen some of them in previous similar incidents.
NOD officials denied any involvement in the attack.
Kasyanov, a former prime minister under President Vladimir Putin and now a Kremlin critic, has been attacked several times in recent months.
A co-chairman of PARNAS, former Deputy Prime Minister Boris Nemtsov, was shot dead near the Kremlin in February 2015.