Russian President Vladimir Putin demanded and won his parliament's approval on Saturday to invade Ukraine, where the new government warned of war, put its troops on high alert and appealed to NATO for help.
Putin's open assertion of the right to send troops to a country of 46 million people on the ramparts of central Europe creates the biggest confrontation between Russia and the West since the Cold War.
Troops with no insignia on their uniforms but clearly Russian - some in vehicles with Russian number plates - have already seized Crimea, an isolated peninsula in the Black Sea where Moscow has a large military presence in the headquarters of its Black Sea Fleet. Kiev's new authorities have been powerless to stop them.
The United States said Russia was in clear violation of Ukrainian sovereignty and called on Moscow to withdraw its forces back to bases in Crimea. It also urged the deployment of international monitors to Ukraine.
Ukrainian Prime Minister Arseny Yatseniuk, leading a government that took power after Moscow's ally Viktor Yanukovich fled a week ago, said Russian military action "would be the beginning of war and the end of any relations between Ukraine and Russia".
Acting President Oleksander Turchinov ordered troops to be placed on high combat alert. Foreign Minister Andriy Deshchytsya said he had met European and U.S. officials and sent a request to NATO to "examine all possibilities to protect the territorial integrity and sovereignty of Ukraine".
The United States will suspend participation in preparatory meetings for a summit of G8 countries in Sochi, Russia, and warned of "greater political and economic isolation", the White House said in a statement after President Barack Obama and Putin held a 90-minute telephone call.
Obama told Putin that if Russia had concerns about ethnic Russians in Ukraine, it should address them peacefully, the White House said.
Putin's move was a direct rebuff to Western leaders who had repeatedly urged Russia not to intervene, including Obama, who just a day earlier had held a televised address to warn Moscow of "costs" if it acted.
Putin told Obama that Russia reserved the right to protect its interests and those of Russian speakers in Ukraine, the Kremlin said.
The Russian forces solidified their control of Crimea and unrest spread to other parts of Ukraine on Saturday. Pro-Russian demonstrators clashed, sometimes violently, with supporters of Ukraine's new authorities and raised the Russian flag over government buildings in several cities.
"This is probably the most dangerous situation in Europe since the Soviet invasion of Czechoslovakia in 1968," said a Western official on condition of anonymity. "Realistically, we have to assume the Crimea is in Russian hands. The challenge now is to deter Russia from taking over the Russian-speaking east of Ukraine."
Putin asked parliament to approve force "in connection with the extraordinary situation in Ukraine, the threat to the lives of citizens of the Russian Federation, our compatriots" and to protect the Black Sea Fleet in Crimea.