Russians in dozens of cities across the country have staged anti-war demonstrations, following a call by advisers close to jailed opposition leader Alexei Navalny.

As of Sunday, the independent OVD-Info monitoring group reports over 2,500 were detained across 49 cities, with more than 800 in Moscow and more than 380 in St. Petersburg.

"The screws are being fully tightened -- essentially we are witnessing military censorship," Maria Kuznetsova, OVD-Info's spokesperson, told Reuters.

Since protests against Russia's invasion of Ukraine began on Feb. 24, nearly 11,000 people have been detained across Russia, according to the group.

Given the number of protests, there was a heavy presence of special police forces patrolling near the Kremlin on Sunday, with Red Square sealed off. A similar scene played out near Palace Square in St. Petersburg — which has seen repeated crowds protest the war.