Ukraine's security council ordered the general staff to immediately put all armed forces on highest alert, the council's secretary Andriy Parubiy announced. The Defense Ministry was ordered to conduct the call-up, potentially of all men up to 40 in a country that still has universal male conscription.
Russian forces who have already bloodlessly seized Crimea – an isolated Black Sea peninsula where most of the population are ethnic Russian and Moscow has a naval base – tried to disarm the small Ukrainian contingents there on Sunday. Some Ukrainian commanders refused to give up weapons and bases were surrounded.
Associated Press journalists say hundreds of unidentified gunmen have just arrived outside Ukraine's infantry base in Privolnoye in its Crimea region.
The convoy includes at least 13 troop vehicles each containing 30 soldiers and four armored vehicles with mounted machine guns. The vehicles – which have Russian license plates – have surrounded the base and are blocking Ukrainian soldiers from entering or leaving it.
Ukrainian soldiers, with clips in their weapons, have positioned a tank at the gate.
Of potentially even greater concern are eastern swathes of the country, where most of the ethnic Ukrainians speak Russian as a native language. Those areas saw violent protests on Saturday, with pro-Moscow demonstrators hoisting flags at government buildings and calling for Russia to defend them.
Putin's declaration that he has the right to invade his neighbor – for which he quickly received the unanimous approval of his parliament – brought the prospect of war to a country of 46 million people on the ramparts of central Europe.
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"President Obama expressed his deep concern over Russia's clear violation of Ukrainian sovereignty and territorial integrity, which is a breach of international law," the White House said after the leaders spoke for 90 minutes on Saturday.
Ukrainian Prime Minister Arseny Yatseniuk, leading a pro-European government that took power after Moscow ally Viktor Yanukovich fled a week ago, said Russian armed action "would be the beginning of war and the end of any relations between Ukraine and Russia".
Foreign Minister Andriy Deshchytsya said he sent a request to NATO to "examine all possibilities to protect the territorial integrity and sovereignty of Ukraine". Ukraine also appealed for help to Britain and the United States, as co-signatories with Moscow to a 1994 accord guaranteeing Ukraine's security after the breakup of the Soviet Union.
NATO ambassadors were due to meet in Brussels on Sunday. Washington has proposed sending monitors to Ukraine under the flags of the United Nations or Organization for Security and Cooperation in Europe, bodies where Moscow would have a veto.
Washington and its allies have suspended plans to attend a G8 summit in Sochi, where Putin had just finished staging his $50 billion winter Olympic games.
"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."
PowerlessIn Crimea, Ukraine's tiny military contingent was powerless to oppose Russian forces, who bore no insignia on their uniforms but drove vehicles with Russian plates and seized government buildings, airports and other locations in the past three days.
Russian news agencies reported that Moscow's troops had disarmed Ukrainians at several small bases.
Igor Mamchev, a Ukrainian navy colonel at a small base near the regional capital Simferopol, told Ukraine's Channel 5 television he had refused to surrender.
"A truck with troops of the Russian Federation, armed with rifles, helmets and bullet-proof vests arrived at our checkpoint and suggested we give up our weapons and accept the protection of the armed forces of the Russian Federation," he said.
"I replied that, as I am a member of the armed forces of Ukraine, under orders of the Ukrainian navy, there could be no discussion of disarmament. In case of any attempt to enter the military base, we will use all means, up to lethal force.
"We are military people, who have given our oath to the people of Ukraine and will carry out our duty until the end."
Ukrainian marines were barricaded into a base in Feodosia, a Crimean port. Russia appealed for them to back the "legitimate" – pro-Russian – regional leadership.
Their commander, Dmytro Delyatytskiy, told Ukraine's Channel 5 by telephone Russian troops had demanded they give up their weapons by 10 am and they refused.
"We have orders," he said. "We are preparing our defenses."
Elsewhere on the occupied peninsula, the Russian forces appeared to be assuming a lower profile on Sunday after the pro-Moscow Crimean leader announced overnight that the situation was now "normalized". Russians had vanished from outside a small Ukrainian guard post in the port of Balaclava that they had surrounded with armored vehicles on Saturday.
The Russian forces had been greeted cheerfully by many, with a wedding party honking its horns and people snapping photos. But there were also some voices of worry.
"No one is attacking us, so why do we need protection – much less armed protection? They are filling up Balaclava with troops," said Olga Karpova, 32.
A barricade in front of the Crimean regional parliament, which the Russians seized on Thursday, had been dismantled. A single armored vehicle with two soldiers drove through the main square, where people were snapping photos.
Putin asked parliament on Saturday 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.
Putin said his request for authorisation to use force in Ukraine would last "until the normalisation of the socio-political situation in that country".
His justification - the need to protect Russian citizens - was the same as he used to launch a 2008 invasion of Georgia, where Russian forces seized two breakaway regions.
In Russia, state controlled media have portrayed Yanukovich's removal as a coup by dangerous terrorists and extremists, funded by the West.
Russian officials have repeatedly described Ukraine's Russian speakers – some of whom have Russian passports – as facing urgent danger. Itar-Tass quoted Russian border guards as saying 675,000 people had fled Ukraine for Russia in the past two months and there were signs of a "humanitarian catastrophe".
In a statement posted online, the Kremlin said that Putin had told Obama "there are real threats to the life and health of Russian citizens and compatriots on Ukrainian territory". Moscow reserved the right to intervene on behalf of Russian speakers anywhere they were threatened, Putin added.
Flags torn downSo far there has been no sign of Russian military action in Ukraine outside Crimea, the only part of the country with a majority that is ethnically Russian and which has often voiced separatist aims at times of tension between Moscow and Kiev.
A bigger risk would be conflict spreading to the rest of Ukraine, where the sides could not be easily kept apart. Demonstrations turned violent in eastern cities on Saturday.
Demonstrators flew Russian flags at government buildings in the cities of Kharkiv, Donetsk, Odessa and Dnipropetrovsk.
The worst violence took place in Kharkiv, where scores of people were wounded when thousands of pro-Russian activists, some brandishing axe handles and chains, stormed the regional government headquarters and fought pitched battles with a smaller number of supporters of Ukraine's new authorities.
In Donetsk, Yanukovich's home city, the local government has called for a referendum on the region's status, a move Kiev says is illegal. A pro-Russian "self-defense" unit, which staged a big protest on Saturday, scheduled another for Sunday.
Thousands of followers, holding a giant Russian flag and chanting "Russia, Russia" marched to the government headquarters and replaced the Ukrainian flag with Russia's.
The Associated Press contributed to this report