A prominent Russian critic of the war is detained after speaking out.

Police officers in masks and camouflage on Wednesday stormed the home of Yevgeny Roizman, perhaps the most vocal critic of the war in Ukraine still freely speaking out inside Russia, and detained him for “discrediting” the Russian Army.

Mr. Roizman, a popular former mayor of the city of Yekaterinburg, near the Ural Mountains, showed calm defiance as the police led him away, according to video footage from the scene. He told reporters waiting outside his apartment door that he was being investigated for “basically one phrase: ‘the invasion of Ukraine.’”

“I say this everywhere, and will say it now,” he said. Referring to his own arrest, he said: “We know all there is to know about our country. This is nothing new.”

Mr. Roizman now faces three years in prison under the censorship law signed by President Vladimir V. Putin in March that made it a crime to call the war in Ukraine a war or an invasion. The Kremlin says it is a “special military operation.” The law has swept up thousands of critics, most of them ordered to pay fines, but some facing prison sentences.

Those targeted by the law include several high-profile opposition figures, like Ilya Yashin and Vladimir Kara-Murza, who were both jailed after continuing to agitate against the war inside Russia even as many other activists fled. Mr. Roizman also decided to stay, pledging he would not “move a millimeter” out of his own country, and railed against the invasion in interviews and in clipped, profanity-laced posts on Twitter.

Mr. Roizman’s arrest is the latest sign that virtually all antiwar dissent has been outlawed in Russia. Analysts had long speculated that the authorities feared cracking down on the jocular and charismatic Mr. Roizman, 59, because of his popularity in Yekaterinburg, one of Russia’s biggest cities. Beyond speaking out against the Kremlin, he raised money for ill children, opened a museum of religious icons, held regular office hours and went jogging with supporters and journalists.

But amid the war and Mr. Putin’s fear of a popular backlash against it, the Kremlin’s calculus appears to have changed.

“The worst thing is when you suddenly see that there is a lot of evil, that evil is winning, that evil is being supported,” Mr. Roizman said in an interview last week seen two million times on YouTube. “Evil can only win when it joins together with a lie.”