Russian President Vladimir Putin won the green light from parliament Saturday to send troops into Ukraine after a bloody three-month uprising swept pro-EU leaders to power while also sparking a pro-Kremlin backlash on the Crimean peninsula.
Ukraine's interim leaders immediately responded by both putting the army on heightened alert and voicing confidence that war will be averted because it would break the two neighbors' historic relations for good.
The stark escalation in what threatens to become the worst crisis in relations between Moscow and the West since the Cold War came as Kalashnikov-wielding militia hoisted the Russian flag over Crimean government buildings and seized control of the peninsula's airports.
Pro-Kremlin rallies also swept several big eastern and southern Ukrainian cities whose cultural links to Moscow stretch back centuries and whose economic survival depends largely on Russian trade.
Putin's shock decision to seek authorization from the upper house of parliament to use force in the ex-Soviet country of 46 million came less than a day after US President Barack Obama warned that any such action would carry "costs" for Moscow.
Putin had issued only one brief statement since Ukraine's parliament on February 22 ousted president Viktor Yanukovych -- who has since fled to Russia -- after a week of carnage in Kiev that claimed nearly 100 lives.
The vast country's bloodiest crisis since its 1991 independence erupted with Yanukovych's decision in November to reject a deal that would have opened Ukraine's door to eventual EU membership in favor of tighter ties with old master Moscow.
The Kremlin said Saturday that Putin had asked the upper house to authorize force "in connection with the extraordinary situation in Ukraine and the threat to the lives of Russian citizens".
"I submit to the Federation Council a request to use the armed forces of the Russian Federation on Ukrainian territory until the normalization of the political situation in that country," the Kremlin quoted Putin as saying.
Putin said Russia also had to protect servicemen of the Black Sea Fleet that is based in Crimea's port town of Sevastopol "fully in line with an international accord".
The Federation Council unanimously approved Putin's request after a lightning-fast debate.
Upper chamber chair Valentina Matviyenko also ordered the Council's foreign affairs committee to ask Putin to recall Russia's ambassador from the United States.
Matviyenko suggested sending in a "limited contingent" -- a phrase that echoed the language used for the Soviet Union's invasion of Afghanistan in 1979.
A senior senator said the ultimate size of the force would be up to Putin.
Ukraine's interim president Oleksandr Turchynov told the nation he had put the country's armed forces on alert and ordered extra security around airports and nuclear power plants.
But Prime Minister Arseniy Yatsenyuk said in the same television broadcast that he was "convinced" Russia would not launch an offensive because "this would mean war and the end of all relations between the two countries."
Ukraine's Defense Minister Igor Tenyukh had earlier said that Russia had already sent 30 armored personnel carriers and 6,000 additional troops into Crimea to help pro-Kremlin militia gain broader independence from the new pro-EU leaders in Kiev.
Putin's move came after an appeal for help from Crimea's newly chosen premier Sergiy Aksyonov -- a ruler not recognized by Kiev and appointed by regional lawmakers after gunmen seized the parliament building in the regional capital Simferopol on Thursday.
"I ask Russian President Vladimir Putin to help in ensuring peace and calm on the territory of Crimea," Aksyonov said in an address broadcast in full by Russian state television.
AFP reporters said several dozen were hurt in the eastern city of Kharkiv when a few hundred people broke away from a pro-Russian crowd of 20,000 and attempted to storm the regional administration center.
More than 10,000 people carrying Russian flags also protested against Kiev's new rulers in the ousted leader's eastern stronghold of Donetsk.
A top Moscow senator said 143,000 Ukrainians had fled to Russia since the crisis first broke out.
Russia's step toward its first war since a five day conflict with neighboring Georgia in 2008 sparked an immediate international outcry.
British Foreign Secretary William Hague said ahead of a visit to Kiev on Sunday that he had summoned the Russian ambassador to register his concerns over Moscow's decision.
"This action is a potentially grave threat to the sovereignty, independence and territorial integrity of Ukraine. We condemn any act of aggression against Ukraine," Hague said.
German Chancellor Angela Merkel said "what is happening in Crimea worries us", while EU foreign affairs chief Catherine Ashton said the 28-nation bloc's foreign minister would hold crisis talks about Ukraine on Monday.
"I deplore today's decision by Russia on the use of armed forces in Ukraine," Ashton said in a statement. "This is an unwarranted escalation of tensions."
UN Secretary General Ban Ki-moon also called for "an immediate restoration of calm and direct dialogue between all concerned to solve the current crisis".
But one top Russian foreign ministry official indicated that Putin may pause before sending troops into Ukraine.
"The agreement that the president received... does not mean that this right will be realized quickly," Russian Deputy Foreign Minister Grigory Karasin told the state-run RIA Novosti news agency.
And Kremlin spokesman Dmitry Peskov told the same agency that "for the moment, this decision has not been taken."
Obama had warned Putin on Friday that "there will be costs for any military intervention in Ukraine".
A senior US official separately told AFP that Obama and some key European leaders could skip the G8 summit in Sochi in June if Moscow's forces became more directly involved in Ukraine.
Kiev's untested new leaders have grappled with the dual dangers of economic collapse and secession by regions that had backed Yanukovych.
The threat of a debt default that Kiev leaders warn could come as early as next week increased further when Russia's state-owned Gazprom -- often accused of being wielded as a weapon by the Kremlin against uncooperative ex-Soviet states -- warned it may be forced to hike the price it charges Ukraine for natural gas.
Ukraine won a one-third discount from Gazprom under a deal signed by Yanukovych with Putin that also saw Russia promise to buy $15 billion of government debt -- a loan frozen after only one payment of $3 billion.
Kiev's new leaders have said they need $35 billion over the coming two years to keep the economy afloat.