The jailed opposition leader has urged Russian and Belarusian citizens to continue defying the authorities.
Russia's jailed opposition leader Alexei Navalny has called on citizens to demonstrate "every day" against the invasion of Ukraine.
The Kremlin critic described the war in Ukraine as "brutal" in a post on social media, and labelled President Vladimir Putin an "obviously insane tsar".
"We -- Russia -- want to be a nation of peace, but few would call us such now," Navalny said.
"But let us at least not become a nation of frightened, silent cowards who pretend not to notice the war of aggression unleashed on Ukraine by our obviously insane tsar," he added.
Navalny also called on citizens in neighbouring Belarus to continue demonstrating, one day after Ukraine said Belarusian forces had entered their territory.
Since last Thursday, more than 6,800 people have been arrested at anti-war demonstrations across Russia, according to the specialist NGO OVD-Info. A further 800 people have also been detained at opposition protests in Minsk and other Belarusian cities.
Navalny has called on protesters to keep defying the authorities and gather in their local square every weekday evening and weekend afternoon.
"I am from the USSR, I was born there, and the catchphrase from there -- from my childhood -- was "fight for peace," Navalny said.
"If we have to fill the prisons and the police cars to stop the war, we will fill the prisons and the police cars."
"I urge everyone to take to the streets and fight for peace," he stated.
Navalny added that he was proud of those Russians that had already been detained during anti-war demonstrations and proclaimed that "Putin is not Russia".
"Every person who has been arrested must be replaced by two people who have come out," read the social media post.
"We must, gritting our teeth and conquering our fear, march out and demand an end to the war."
Russia's opposition leader has been imprisoned near Moscow since his return to Russia in January 2021. Navalny had spent several months recovering in Germany from a nerve agent poisoning that he blames on the Kremlin. Moscow has denied the allegations.
Navalny is currently serving a two-and-a-half-year prison sentence for a 2014 fraud charge that he says is politically motivated.
He is also currently on trial on fresh corruption charges that could keep him behind bars for at least another ten years.
Many of Navalny's allies and supporters have been forced into exile after Russia declared his AntiCorruption Foundation an "extremist" organisation last year.