MARGARET WARNER:

The demonstrations were the biggest show of defiance against President Vladimir Putin since 2012. Tens of thousands rallied in 99 cities, from St. Petersburg and Moscow in the west, to Chita in Siberia, to the far Pacific coast port of Vladivostok.

Navalny generated the protests, in part, to demand the resignation of Prime Minister and former President Dmitry Medvedev. Navalny released a video earlier this month showcasing myriad mansions, yachts and vineyards Medvedev allegedly has amassed. Hundreds of protesters were arrested yesterday in Moscow and elsewhere.

Today, the European Union called for their release. A Putin spokesman said: "The Kremlin respects people's civic stance and their right to voice their position. We can't express the same respect to those who consciously misled people and provoked illegal actions."

White House spokesman Sean Spicer said today: