Ukraine has issued an arrest warrant for President Viktor Yanukovych as Russia declared the situation in its neighbour was a "real threat" to its interests.
Ukraine's acting interior minister said Mr. Yanukovych, whose exact whereabouts are unknown, and other officials are wanted by police for "mass murder" of peaceful citizens.
Arsen Avakov said the deposed leader had left a private residence in Balaclava, in the Russian-speaking Crimea region, for an unknown destination with one of his aides.
Spokesmen for the Interior Ministry and Security Service in Crimea said they had no information to support reports from opposition politicians and a news website that the president had been arrested or seen with Russian marines there.
Western governments are scrambling to provide support for Ukraine amid fears that Russia could send in troops to intervene in support of its ousted ally.
Journalist Sabra Ayres is a journalist based in the capital Kiev.
Russian Prime Minister Dmitry Medvedev, however, said their backing of the interim government was an "aberration".
"We do not understand what is going on there. There is a real threat to our interests and to the lives of our citizens," Russian news agencies quoted him as saying.
"There are big doubts about the legitimacy of a whole series of organs of power that are now functioning there".
Russian Foreign Minister Sergei Lavrov said Ukraine's opposition "had in effect seized power in Kiev, refused to disarm and continued to place its bets on violence".
Russia's Economy Minister has also warned Kiev against signing a partnership agreement with the EU - threatening to hike import duties if Ukraine looks to forge closer ties with the west.
The European Union foreign policy chief Catherine Ashton is due to meet Ukraine's interim leaders in Kiev to discuss plans "for a lasting solution to the political crisis and measures to stabilise the economic situation".
Her arrival comes after Moscow recalled its ambassador in Ukraine, Mikhail Zurabov, on Sunday for consultations on the "deteriorating situation" in the country.
US warns Russia of "grave mistake"
In the wake of the violence on the streets of Kiev, US national security adviser Susan Rice warned Russia that it would be committing a "grave mistake" if it sends troops into Ukraine.
Russia hit back, with the Interfax news agency quoting foreign ministry sources as saying Ms. Rice should urge her own country's leadership to avoid using force.
Britain has joined the EU and US in pledging financial assistance as the West prepares for the possibility that Mr Putin could pull the plug on a US$15 billion (€10.8) deal to prop up Ukraine's ailing economy.
Ukraine's Finance Ministry says it needs US$35 billion (€25.5) to get through 2014 and 2015 and a senior European Commission source said talks have taken place with Japan, China, Canada, Turkey and the US, with efforts being made to keep Russia engaged.
Russia's financial backing was seen as a reward for Mr. Yanukovych's decision last November to spurn an EU trade deal and opt for closer links with Moscow - a move that sparked protests in Kiev.
A US official said Russian Finance Minister Anton Siluanov had refused to confirm that the next installment would be paid following the revolution, during discussions on the sidelines of the G20 meeting in Sydney.
Discussions of Ukraine's future come after the country's interim leadership set a course for European integration now that Mr. Yanukovych has been ousted.
Acting President Oleksandr Turchyno - a close ally of former Prime Minister Yulia Tymoshenko - said Ukraine would seek relations with Russia on a "new, equal and good-neighbourly footing."
Parliament has until Tuesday to form a new unity government.
Former Georgian President Mikheil Saakashvili - who led his country's pro-West "Rose Revolution" in 2003 - joined protesters in Independence Square on Sunday night.
Mr. Yanukovych was deposed by parliament on Saturday after months of bloody unrest - with 82 people killed in clashes between protesters and riot police. He has been refused permission to fly out of Ukraine and has said he will not resign.
Opposition leader Vitali Klitschko said "It's a remarkable situation when the most sought-after character in the country is the president of Ukraine, who is hiding and doing everything to avoid responsibility. I would like to underline, the hundreds of victims are the responsibility of Yanukovych".
CCTV has emerged apparently showing items being removed from the Presidential compound on Friday and a figure boarding a helicopter before it flew away. Financial documents found in the abandoned compound also detailed Mr Yanukovych's lavish spending, often on unusual items.
Among the documents was a receipt for US$12 million (€8.7) in cash, and US$110,000 (€80,000) spent on curtains in a room called the "Knight's Hall". Some US$1.5 million (€1,100,000) was spent on plants, US$115,000 (€84,000) for a statue of a "running boar", and there was a receipt for a US$4,000 (€3,000) "bribe".
You can see pictures of Mr Yanukovych's estate here.
