Despite the threat of years-long prison terms, thousands of Russians joined anti-war rallies across the country on Sunday in a striking show of the pent-up anger in Russian society about President Vladimir V. Putin’s invasion of Ukraine.

Police reported close to 4,000 arrests across the country — the highest nationwide total officially reported in any single day of protest in recent memory. An activist group that tracks arrests, OVD-Info, reported detentions in 49 different Russian cities.

Video from independent Russian news outlets covering the protests showed throngs of people chanting “No to war!” on St. Petersburg’s central avenue, Nevsky Prospekt, and on Moscow’s Manezhnaya Square, just outside the Kremlin walls.

In the city of Kaliningrad near the Baltic Sea, a woman protesting the war was recorded in a video posted on Twitter telling a police officer that she had survived the Nazi siege of Leningrad.

“Are you here to support the fascists?” the officer responded, repeating the Kremlin narrative about the war in Ukraine, before calling over other policemen and telling them: “Arrest them all.”

The thousands of Russians who protested on Sunday represented only a slice of those furious over the invasion. Thousands more fled the country in the last 10 days, as their savings evaporated amid the collapse of the ruble and the West’s crushing sanctions.

“There is no more Russia,” Anton Dolin, one of Russia’s best-known film critics, wrote on Sunday, announcing his departure. “We are suffering a catastrophe — no, not an economic or political one. This is a moral catastrophe.”