A tense standoff was in place in the Crimea crisis tonight as Russian forces continued to surround a Ukrainian army base.
Ukrainian authorities responded to the surprise Russian takeover of Crimea by mobilising its military. There was no justification for the Russian action, Ukrainian Prime Minister Arseniy Yatsenyuk said as he warned: "We are on the brink of disaster".
"We believe that our western partners and the entire global community will support the territorial integrity and unity of Ukraine," he said in Kiev.
Outrage over Russia's tactics mounted in world capitals, with US Secretary of State John Kerry calling on President Vladimir Putin to pull back from "an incredible act of aggression".
Fearing that Europe's borders were being rewritten by force, world leaders rushed to find a diplomatic solution to reverse what had already happened on the ground: Russia captured the Black Sea peninsula yesterday without firing a shot.
NATO held an emergency meeting in Brussels, Britain's foreign minister flew to Kiev to support its new government and the US, France and Britain debated the possibility of boycotting the next Group of Eight economic summit, being held in June at Sochi, the host of Russia's successful Winter Olympics.
In Kiev, Moscow and other cities, thousands of protesters took to the streets to either decry the Russian occupation or celebrate Crimea's return to its former ruler.
Mr Kerry, interviewed in a series of US television news shows, talked about boycotting the G-8 summit, as well as possible visa bans, asset freezes and trade and investment penalties against Russia. He said all the foreign ministers he had talked to were prepared "to go to the hilt" to isolate Russia.
Still, politicians were treading carefully, knowing it was a delicate time for Europe.
"We are on a very dangerous track of increasing tensions," German Foreign Minister Frank-Walter Steinmeier said. "(But) it is still possible to turn around. A new division of Europe can still be prevented."
So far, however, Ukraine's new government and other countries have been powerless to counter the stealth Russian military tactics. Armed men in uniforms without insignia have moved freely about Crimea for days, occupying airports, smashing equipment at an air base and besieging a Ukrainian infantry base.
Mr Putin has defied calls from the West to pull back his troops, insisting that Russia has a right to protect its interests and those of Russian-speakers in Crimea and elsewhere in Ukraine. His confidence is matched by the knowledge that Ukraine's 46 million people have divided loyalties between Russia and Europe. While much of western Ukraine wants closer ties with the 28-nation European Union, its eastern and southern regions like Crimea look to Russia for support.
Russia has long wanted to reclaim the lush Crimean Peninsula, which was part of its territory until 1954. Russia's Black Sea Fleet pays Ukraine millions every year to be stationed at the Crimean port of Sevastopol and nearly 60% of Crimea's residents identify themselves as Russian.
The new Ukrainian government came to power last week following months of pro-democracy protests against a pro-Russian president, Viktor Yanukovych, and his decision to turn Ukraine toward Russia instead of the EU. Mr Yanukovych fled to Russia after more than 80 people were killed in the protests. He insists he's still president.
Ukraine's acting president, Oleksandr Turchynov, has ordered Ukraine's armed forces to be on full alert and stepped-up security at nuclear power plants, airports and other strategic infrastructure. Today the government called reservists into action but no overt military actions by Ukraine were seen.
Mr Turchynov also appointed 18 new regional governors, including two oligarchs in the eastern cities of Dneprotrovsk and Donetsk, as big business and the new Ukrainian government banded together against Russia.
Russian troops, meanwhile, pulled up to the Ukrainian military base at Perevalne on the Crimean Peninsula in a convoy that included at least 13 trucks and four armoured vehicles with mounted machine guns. The trucks carried 30 soldiers each and had Russian licence plates.
In response, a dozen Ukrainian soldiers, some with clips in their rifles, placed a tank at the base's gate, leaving the two sides in a tense standoff. It appeared to be the first known case of outmatched Ukrainians standing up to Russian military might.
Unidentified soldiers were also seen cutting power to the headquarters of the Ukrainian Naval forces in Crimea - whose own commander defected today and pledged his allegiance to "the people of Crimea".
A Ukrainian security official later said the head of the country's Black Sea fleet - Admiral Denis Berezovsky - had been dismissed and faces a treason investigation after declaring allegiance to the pro-Russian government of the Crimea region and offering no resistance to the Russian troops.
The speaker of Crimea's legislature, Vladimir Konstantinov, said local authorities do not recognise the new government in Kiev. He said a planned referendum on March 30 would ask voters about the region's future status.
A convoy of hundreds of Russian troops was also seen heading toward Simferopol, the regional capital of Crimea.
In Brussels, NATO's secretary general said Russia had violated the UN charter with its military action in Ukraine, and he urged Moscow to "de-escalate the tensions." NATO chief Anders Fogh Rasmussen spoke before a meeting of the alliance's political decision-making body to discuss the crisis.
Ukraine is not a NATO member, meaning the US and Europe are not obligated to come to its defence. But Ukraine has taken part in some alliance military exercises.
President Barack Obama talked with Mr Putin by telephone for 90 minutes yesterday and expressed his "deep concern" about "Russia's clear violation of Ukrainian sovereignty".
Mr Obama warned that Russia's "continued violation of international law will lead to greater political and economic isolation".
Mr Putin defended Russia's action against "ultranationalist forces" in Ukraine during a phone conversation with German Chancellor Angela Merkel.
A Kremlin statement posted online said Mr Putin spoke with Mrs Merkel by phone today, and that he "directed her attention to the unrelenting threat of violence" to "Russian citizens and the whole Russian-speaking population".
The statement said Mr Putin had stressed that measures taken by Russia so far were "fully adequate".
Later the German government said Mr Putin had accepted a proposal by Mrs Merkel for a fact-finding mission and a "contact group" on Ukraine that could be led by the Organisation for Security and Cooperation in Europe.
Government spokesman Georg Streiter said the proposal came during the telephone conversation in which Mrs Merkel accused Mr Putin of breaking international law with the "unacceptable Russian intervention in Crimea".
The "contact group," which Mrs Merkel's foreign minister floated, would be aimed at getting dialogue going between Russia and Ukraine.