Russian authorities have recently tightened the screws on critics of the military operation.
Vladimir Kara-Murza, one of the main opponents of the Kremlin and its offensive in Ukraine still living in Russia, was arrested Monday near his home, his lawyer said.
"I learned of his arrest less than 10 minutes ago, I will join him," lawyer Vadim Prokhorov told the Interfax news agency.
The reasons for the arrest were not immediately known and there was no immediate comment from the authorities, but Kara-Murza has repeatedly criticised Moscow's military intervention in Ukraine.
Russian authorities have recently tightened the screws on critics of the military operation.
Publishing information about the military deemed false by the government is punishable by up to 15 years in prison.
The Russian intervention has been accompanied by an all-out crackdown in Russia, with thousands of protesters arrested, as well as the closure of NGOs, independent media and several social networks.
Kara-Murza, 40, is a former journalist close to opposition leader Boris Nemtsov, who was assassinated near the Kremlin in 2015, and Mikhail Khodorkovsky, a former oligarch turned critic of President Vladimir Putin.
Kara-Murza claims he was poisoned twice, in 2015 and 2017, because of his political activities.
Despite the incidents, he continues to live in Russia, where many opposition figures have opted
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