KIEV/BALACLAVA, Ukraine - Ukraine mobilized for war on Sunday, after Russian President Vladimir Putin declared he had the right to invade, creating the biggest confrontation between Moscow and the West since the Cold War.

"This is not a threat: this is actually the declaration of war to my country," said Ukraine's Prime Minister Arseny Yatseniuk, head of a pro-Western government that took power when Russian ally Viktor Yanukovich fled last week.

Putin obtained permission from his parliament on Saturday to use military force to protect Russian citizens in Ukraine, spurning Western pleas not intervene.

Russian forces have already bloodlessly seized Crimea - an isolated Black Sea peninsula where Moscow has a naval base. On Sunday they surrounded several small Ukrainian military outposts there and demanded the Ukrainian troops disarm. Some refused, although no shots were fired.

Russia has staged war games with 150,000 troops along the land border, but so far they have not crossed. However, pro-Russian demonstrators have marched in the east of the country and have raised Russian flags over government buildings in several cities, in what Kiev says is a move orchestrated by Moscow to justify a wider invasion.

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 a call-up of reserves - theoretically all men up to 40 in a country with universal male conscription, though Ukraine would struggle to find extra guns or uniforms for significant numbers of them.

"If President Putin wants to be the president who started the war between two neighboring and friendly countries, between Ukraine and Russia, so: he has reached this target within a few inches. We are on the brink of disaster," Yatseniuk said in televised remarks in English, appealing for Western support.

At Kiev's Independence Square, where anti-Yanukovich protesters had camped out for months, thousands demonstrated against Russian military action. Speakers delivered rousing orations and placards read: "Putin, hands off Ukraine!"

Oleh, an advertising executive cooking over a big open fire at the square where he has been camped for three months, said: "If there is a need to protect the nation, we will go and defend the nation.... If Putin wants to take Ukraine for himself, he will fail. We want to live freely and we will live freely."

Of potentially even greater concern than Russia's seizure of majority ethnic Russian Crimea are eastern swathes of the country, where most ethnic Ukrainians speak Russian as a native language.

Those areas saw more demonstrations on Sunday after violent protests on Saturday, and for a second day pro-Moscow demonstrators hoisted flags at government buildings and called for Russia to defend them. Kiev said Russia had sent hundreds of its citizens across the border to stage the protests.

Putin's declaration that he has the right to invade his neighbor - for which he quickly received the unanimous approval of his senate - brought the prospect of war to a country of 46 million people on the ramparts of central Europe.

"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.

Ukranian-Americans rally White House for supportPlay videoUkranian-Americans rally White House for support

Ukraine has appealed for help to NATO, and directly 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 met in Brussels to discuss their next steps. Secretary General Anders Fogh Rasmussen accused Russia of threatening peace and security in Europe.

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.

So far, the Western response has been largely symbolic. Obama and other leaders suspended plans to attend a G8 summit in Sochi, where Putin has just finished staging his $50 billion winter Olympic games. Some countries recalled ambassadors.

"This is probably the most dangerous situation in Europe since the Soviet invasion of Czechoslovakia in 1968," said a Western official. "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."

Ukraine's tiny armed forces would be no match against the might of its superpower neighbor. Britain's International Institute of Strategic Studies estimates Kiev has fewer than 130,000 troops under arms, with planes barely ready to fly and few spare parts for a single submarine.

Russia, by contrast, has spent billions under Putin to upgrade and modernize the capabilities of forces that were dilapidated after the breakup of the Soviet Union. Moscow's special units are now seen as equals of the best in the world.

Ukraine puts forces on combat alert, warns of warPlay videoUkraine puts forces on combat alert, warns of war

In Crimea, Ukraine's tiny contingent made no attempt to oppose the Russians, 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. Kiev said its troops were encircled at least three places.

Igor Mamchev, a Ukrainian navy colonel at a small base near the regional capital Simferopol, told Ukraine's Channel 5 television a truckload of Russian troops had arrived at his checkpoint and told his forces to lay down their arms.

"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."

Dmytro Delyatytskiy, commander of Ukrainian marines barricaded into a base in the Crimean port of Feodosia, told the same television station by telephone he had refused a Russian demand that his troops give up weapons by 10 a.m.

"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.

A barricade in front of the Crimean regional parliament had been dismantled. A single armored vehicle with two soldiers drove through the main square, where people snapped photos.

Putin's 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 portray Yanukovich's removal as a coup by dangerous extremists funded by the West and there has been little sign of dissent with that line.

Putin told Obama "there are real threats to the life and health of Russian citizens and compatriots on Ukrainian territory", according to the Kremlin's readout of the phone call. Moscow reserved the right to intervene on behalf of Russian speakers anywhere they were threatened, Putin added.

So far there has been no sign of Russian military action outside Crimea, but Kiev officials accused Moscow of being behind the pattern of violent protests in eastern cities.

Pro-Moscow demonstrators flew Russian flags on Saturday and Sunday at government buildings in cities including Kharkiv, Donetsk, Odessa and Dnipropetrovsk. In places they clashed with anti-Russian protesters and guards defending the buildings.

Ukrainian parliamentarian Hrygory Nemyriya, a spokesman to foreign journalists for the new authorities, said the pro-Moscow marchers were sent from Russia.

The worst violence took place in Kharkiv, where scores of people were hurt on Saturday when thousands of pro-Russian activists, some brandishing axe handles and chains, stormed the regional government and fought pitched battles with a smaller number of supporters of Ukraine's new authorities.

In Donetsk, Yanukovich's home city, the local government building was flying the Russian flag for the second day on Sunday. The local authorities have called for a referendum on the region's status, a move Kiev says is illegal. A pro-Russian "self-defense" unit held a second day of mass protests, attracting about 1,000 demonstrators carrying Russian flags.

Ludmila Petrova, 35, described the new authorities in Kiev as "slaves of the European Union" and said she favored Putin's declaration of the right to intervene.

"Maybe this will stop the hotheads in Kiev from bringing war to the Don basin and the Crimea. Maybe now they will think there is someone willing to defend these people." (Reuters)