Russian President Vladimir Putin has defended the separatist drive in the disputed Crimean Peninsula as in keeping with international law, as Ukraine's prime minister vowed not to relinquish "a single centimetre" of his country's territory.
Over the weekend, the Kremlin beefed up its military presence in Crimea, a part of Ukraine since 1954, and pro-Russia forces keep pushing for a vote in favour of reunification with Moscow in a referendum the local parliament has scheduled for next Sunday.
US President Barack Obama has warned that the March 16 vote would violate international law. But in Moscow, Mr Putin made it clear that he supports the referendum in phone calls with German Chancellor Angela Merkel and British Prime Minister David Cameron.
"The steps taken by the legitimate leadership of Crimea are based on the norms of international law and aim to ensure the legal interests of the population of the peninsula," said Mr Putin, according to the Kremlin.
Following an extraordinary Sunday meeting of the Ukrainian government, Prime Minister Arseniy Yatsenyuk announced that he will fly to the United States this week for high-level talks on "resolution of the situation in Ukraine," the Interfax news agency reported.
"Our country and our people are facing the biggest challenges in the history of modern independent Ukraine," the prime minister said earlier in the day. "Will we be able to deal with these challenges? There should only be one answer to this question and that is: yes."
In an emotional climate of crisis, Ukraine solemnly commemorated the 200th anniversary of the birth of its greatest poet, Taras Shevchenko, a son of peasant serfs who is a national hero and is considered the father of modern Ukrainian literature.
"This is our land," Mr Yatsenyuk told a crowd gathered at the Kiev statue to Shevchenko. "Our fathers and grandfathers have spilled their blood for this land. And we won't budge a single centimetre from Ukrainian land. Let Russia and its president know this."
"We're one country, one family and we're here together with our kobzar (bard) Taras," said acting President Oleksandr Turchynov.
Later, Ukrainians in the tens of thousands massed in the Kiev's centre for a multi-faith prayer meeting to display unity and honour Shevchenko. One of the speakers, former imprisoned Russian tycoon Mikhail Khodorkovsky, almost burst into tears as he implored the crowd to believe that not all Russians support their country's recent actions in Ukraine.
"I want you to know there is a completely different Russia," Mr Khodorkovsky said.
Crimea, a strategic peninsula in the Black Sea, has become the flashpoint in the battle for Ukraine, where three months of protests sparked by President Victor Yanukovych's decision to ditch a significant treaty with the 28-nation European Union after strong pressure from Russia led to his downfall. A majority of people in Crimea identify with Russia, and Moscow's Black Sea Fleet is based in Sevastopol, as is Ukraine's.
In Simferopol, Crimea's capital, a crowd of more than 4,000 people turned out to endorse unification with Russia. On Lenin Square, a naval band played World War Two songs as old women sang along, and dozens of tricolour Russian flags fluttered in the cold wind.
"Russians are our brothers," Crimean Parliament speaker Vladimir Konstantinov said. He asked the crowd how it would vote in the referendum a week hence.
"Russia! Russia!" came the loud answer.
"We are going back home to the motherland," said Mr Konstantinov.
Across town, at a park where a large bust of Shevchenko stands, around 500 people, some wearing yellow-and-blue Ukrainian flags on their shoulders like capes, came out to oppose unification with Russia.
They chanted "No to the referendum!" and "Ukraine!". People handed out leaflets, one of which listed the economic woes that joining Russia would supposedly cause.
"We will not allow a foreign boot that wants to stand on the heads of our children," said one of the speakers, Alla Petrova. "The people are not scared. We are not scared to come out here and speak."
Some pro-Russians drove by, shouting "Moscow, Moscow!" from their cars, but there was no trouble.
British Foreign Secretary William Hague described Russia's entering Crimea as a "big miscalculation".
He also said the March 16 referendum was happening "ridiculously quickly". Mr Hague added: "The world will not be able to regard that as free or fair."
During his conversations with Mr Cameron and Ms Merkel, Mr Putin criticised the Western leaders for what he said was their failure to press the new government in Kiev to curb ultra-nationalist and radical forces.
But the Kremlin also said that despite their differences, the three leaders expressed an interest in reducing tensions and normalising the situation in Ukraine as soon as possible.