Prime Minister Mykola Azarov of Ukraine says an investment deal with Russia has helped the country avoid bankruptcy.
Mr Azarov said in Kiev on Wednesday the package from Russia would provide stability.
Russia has agreed to buy 11 billion euros of government bonds and lower the price of gas by about a third.
The opposition has demanded to know what Ukraine offered Russia in return.
Pro-EU protesters are holding rallies in Kiev - where they occupy Independence Square - and other cities in western and central Ukraine.
Critics say President Viktor Yanukovych has sold out to Russia and are calling for him and the government to step down.
"What would have awaited Ukraine? The answer is clear - bankruptcy and social collapse," Mr Azarov said.
"What a present for New Year that would be for the people of Ukraine.''
There was no way Ukraine could have signed an EU agreement as Kiev would have had to accept unfeasibly stringent IMF conditions for economic reform, he added.
Meanwhile, German chancellor Angela Merkel told parliament in Berlin she regretted Ukraine's decision - but said the offer of an EU trade pact was still open.
The BBC reports Ukraine urgently needs to cover an external funding gap of up to 12.3 billion euros next year to avoid defaulting on its debts.
The country relies on imports of Russian gas: 75% of Ukraine's engineering exports go to Russia.