Armed men patrol as a man holds a Soviet Navy flag in the centre of the Crimean city of Simferopol.
Russian parliament has granted President Vladimir Putin permission to use the country's military in Ukraine.
Saturday's unanimous vote formalized what Ukrainian officials described as an ongoing deployment of Russian troops in the strategic region of Crimea. It also raised the possibility that Moscow could send its military elsewhere in Ukraine.
Putin's call came as pro-Russian demonstrations broke out in Ukraine's Russian-speaking east, where protesters raised Russian flags and beat up supporters of the new Ukrainian government.
Russia's move sharply raised the stakes in the conflict following the ouster of Ukraine's pro-Russian president last week by a protest movement aimed at turning Ukraine toward the European Union and away from Russia. President Barack Obama warned Moscow on Friday "there will be costs" if Russia intervenes militarily.