President Vladimir Putin ordered Russia's first wartime mobilization since World War II on Wednesday, shocking citizens with what Western countries described as an act of desperation in the face of a losing war.

Putin made the announcement in a televised address in which he also announced moves to annex swaths of Ukrainian territory and threatened to use nuclear weapons to defend Russia, declaring: "It's not a bluff."

Flights out of Russia quickly sold out, and jailed opposition leader Alexei Navalny called for mass demonstrations against the mobilization. Russians said some people were already receiving call-up notices, and police were barring men from leaving one city in the south.