Jailed Kremlin critic Alexei Navalny has called on Russians around the world to protest President Vladimir Putin’s invasion of Ukraine, issuing tweets saying that people must not just “be against the war” but they must also “fight against” it.

“We — Russia — want to be a nation of peace. Alas, few people would call us that now,” Navalny said Tuesday in a message posted by a spokesperson on his Twitter account. He urged people to take to the streets and head to Russian embassies in their countries to oppose what he called “the aggressive war against Ukraine unleashed by our obviously insane czar.”

“It’s the third decade of the 21st century, and we are watching news about people burning down in tanks and bombed houses,” he tweeted to his 2.7 million followers, urging them to take a stand against the Russian government. “We are watching real threats to start a nuclear war on our TVs.”

“Each arrested person must be replaced by two newcomers,” he continued, citing the risks that Russians face when protesting against Putin.

Thousands of horrified Russians have already taken to the streets in recent days to protest the invasion, a rare display of anger in a country where spontaneous demonstrations are illegal and attendees face fines and imprisonment.

An estimated 6,000 protesters have been arrested across more than 50 cities in Russia, the Associated Press reported Sunday, citing data from rights group OVD-Info. Some chanted “No to war!” while others spoke of their anger at the government’s bombardment of Ukraine.

Navalny rose to international prominence for his opposition to Putin and was poisoned in Russia in 2020 by a banned chemical weapon that nearly killed him, according to a German laboratory.

Broadly known as one of Putin’s sharpest critics, the 45-year-old made a recovery in Germany after being placed in a medically induced coma. He later returned to Russia, where he was detained and imprisoned. Navalny blamed Putin for the attack on him using the nerve agent Novichok. Putin denied involvement.

Following the poisoning, the Biden administration announced sanctions last year against seven senior Russian government figures, although the penalties were not directly aimed at Putin.

Russia branded the U.S. sanctions “a hostile anti-Russian attack,” as the United States continued to demand Navalny’s release.

One month earlier, European Union foreign ministers agreed to impose sanctions on top Russian officials following Navalny’s imprisonment and announced asset freezes and a travel ban.

According to analysts, the poisoning of Navalny demonstrates just how significant a rival he is considered to be by Putin and Russian officials.

In October, Navalny was awarded the prestigious European human rights Sakharov Prize in recognition of his work.