Moscow police opened a criminal inquiry on Monday after anti-Kremlin opposition activist and political blogger Yegor Zhukov was beaten and left bloodied outside his home at the weekend.
The 22-year-old student, who was a leading figure in large anti-government protests last year, was taken to hospital late on Sunday after being assaulted by two people outside his apartment in western Moscow, his allies said on social media.
They published pictures of his bloodied face covered in bruises and gashes.
He was among more than 1,000 people detained in Moscow in July 2019 in one of the biggest crackdowns on anti-Kremlin demonstrators in recent years. He and other protesters had taken to the streets to call for free elections.
A Russian court last year found him guilty of inciting extremism on his YouTube channel and banned him from using the Internet for two years, a move he decried as politically motivated.
The Moscow police force said it was seeking to identify and detain Zhukov's assailants, who his allies said had fled the scene of the attack on scooters. If convicted of assault, Zhukov's attackers could face up to two years in jail.
Kremlin spokesman Dmitry Peskov said he hoped Zhukov's attackers would be found and punished.
Peter Stano, a spokesman for the European Commission, condemned the attack on Zhukov at a news briefing in Brussels.
"We wish him a speedy recovery and we expect that those responsible for this brutal and cowardly attack will be brought to justice as soon as possible," said Stano.
He said it looked to be part of what he called "a very negative trend on human rights in Russia" and attacks on people who expressed dissenting views.
Zhukov said on Sunday that due to his Kremlin opposition activities he was expelled last week from the Moscow university where he had been studying.
Another Russian opposition activist, Alexei Navalny, remains in a medically induced coma in a German hospital after what his allies say was a poisoning.