In a long thread of posts on Twitter on March 2, the Kremlin critic slammed President Vladimir Putin as an "obviously insane czar" for unleashing war on Ukraine, while reminding Russians of the Soviet-era mantra of "struggle for peace."

"Let's not become a nation of frightened silent people, of cowards who pretend not to notice the aggressive war unleashed by our obviously insane czar against Ukraine," Navalny wrote. "Putin is not Russia."

Navalny, Putin's most vocal critic, was jailed in 2021 upon his return to Russia from Germany, where he was recovering from a near-fatal poisoning while traveling in Siberia from what Western laboratory tests established was a Soviet-style nerve agent.

He has blamed Putin directly for the attempt on his life, a claim the Kremlin has repeatedly denied.

Sporadic protests have been seen across Russia since the invasion began on February 24, with police cracking down quickly on any dissent.

According to OVD-Info, a nonprofit that monitors police arrests nationwide, 6,835 people have been detained so far for anti-war actions in Russia.

"We must, gritting our teeth and overcoming fear, come out and demand an end to the war.... Each arrested person must be replaced by two newcomers. If -- in order to stop the war -- we have to fill prisons and paddy wagons with ourselves, we will fill prisons and paddy wagons with ourselves," Navalny wrote.

"Everything has a price and now, in the spring of 2022, we must pay this price. No one else. Let's not 'be against the war.' Let's fight against the war," he added.