Russia has vetoed a draft UN resolution criticising Sunday's secession referendum in Ukraine's Crimea region - the only Security Council member to vote against the measure.
China, regarded as a Russian ally on the issue, abstained from the vote.
Western powers criticised Russia's veto over the referendum, which will ask Crimeans if they want to rejoin Russia.
Meanwhile, Kiev has accused Russian forces of seizing a village just north of Crimea and demanded they withdraw.
Ukraine's foreign ministry said 80 military personnel backed by four helicopter gunships and three armoured vehicles had taken the village of Strilkove.
An unnamed Russian official quoted by Pravda-Ukraine said they had taken action to protect a gas distribution station from "terrorist attacks".
Russia intervened in the Crimean peninsula after the fall of Ukraine's pro-Moscow President Viktor Yanukovych on 22 February.
Mr Yanukovych had sparked months of unrest across Ukraine by pulling out of a deal on closer ties with the European Union, and later opting for closer ties with Russia.
The Crimean region was part of Russia until 1954 and most of its residents are ethnic Russians, many of whom would prefer to be governed by Moscow rather than Kiev.
Russia's Black Sea fleet is also still housed in Crimea.
But Russia has signed agreements promising to uphold Ukraine's territorial integrity.
Crimea's regional parliament instigated the secession referendum after MPs voted overwhelmingly to support rejoining Russia.
But the national parliament in Kiev ruled the referendum unconstitutional, and earlier on Saturday voted to disband the regional assembly.
At the United Nations, 13 members of the Security Council backed a resolution that called for all nations to respect Ukraine's territorial integrity and condemned the referendum as illegal.
Western diplomats had expected Russia to veto the document, but got what they wanted when China abstained, says the BBC UN correspondent Nick Bryant.
China and Russia usually work in tandem at the Security Council.
But Beijing is sensitive about issues of territorial integrity, because of fears it could send a message to its own restive regions of Tibet and Xinjiang, our correspondent says.
America's UN envoy Samantha Power said it was a "sad and remarkable moment" and labelled Russia "isolated, alone and wrong".
She said Sunday's referendum was "illegal, unjustified and divisive" and would have no effect on the legal status of Crimea.
Russian envoy Vitaly Churkin said the referendum was necessary to fill the "legal vacuum" since Ukraine's "coup d'etat" last month.
Earlier in Moscow, tens of thousands rallied against Russia's actions in Ukraine, the biggest such protest in two years.
As many as 50,000 attended the rally, with protesters shouting: "Hands off Ukraine."
One man told the BBC he felt Russia was turning back to the days of Soviet dictator Joseph Stalin.
Nearby, some 15,000 supporters of President Vladimir Putin came out to support the Crimean referendum.
Many of them wore identical red outfits and carried Russian and Soviet flags
"We are for friendship of the Russian and Ukrainian peoples. We want to say a firm 'No' to the fascist junta that came to power in Ukraine," one man said.