Others are pursuing subtler forms of protest in the hope they won’t result in immediate arrest. Some are covering Moscow’s walls with the simple, straightforward call: “No to war.” (The messages are scrubbed away by officials, only to reappear overnight.) Others are laying flowers on the Kyiv monument near Red Square, which commemorates the bravery of its defenders in World War II.

Beyond the streets, people are busy too. A petition condemning the war has already received more than a million signatures, and architects, medical workers, university students and even priests in the normally acquiescent Russian Orthodox Church are signing open letters demanding that it stop immediately. Big names like Yuri Dud, Russia’s most prominent video blogger, the popular singer Valery Meladze and even several State Duma members and top oligarchs have publicly spoken out in an unlikely chorus of voices.

A mass antiwar movement is still a way off. But these are, amid the gloom, promising signs. As the country continues to bomb and terrify Ukraine, more and more Russians may wake up to something only a few dare to say publicly: That Mr. Putin is an existential danger not only to themselves but also to the world. And he must be stopped.