Russia has vetoed a U.N. Security Council resolution that would have urged countries not to recognize the results of a Sunday referendum in Ukraine's Crimea region.
Russia was the only Security Council member to vote against the measure on Saturday, while China abstained. The 13 other council members backed the measure.
China's decision to decline to follow Russia and veto the measure could indicate further isolation for Moscow for its support of the Crimean referendum.
British Ambassador Mark Lyall Grant said the "message" from Saturday's vote was that "Russia stands isolated" in the Security Council and international community.
The referendum gives residents of Crimea two choices: join Russia or support a significant strengthening of Crimea's autonomy within Ukraine.
Shortly before the vote, Russian Ambassador to the U.N. Vitaly Churkin told the council the resolution ran counter to the principles of international law.
The measure would have affirmed Ukraine's territorial integrity by declaring Sunday's Crimea referendum could "have no validity."
After the vote, China's U.N. Ambassador Liu Jieyi said his country's only option was to abstain because the draft U.N. resolution would only further complicate the situation in Ukraine.
U.S. Ambassador Samantha Power reacted to the measure's defeat, saying "This is a sad and remarkable moment."
In another development Saturday, the Ukrainian military said it had repelled an attempt by Russian forces to enter a region near Crimea. Ukraine's defense ministry said its military used aircraft and ground forces to push back Russian forces.
In Moscow Saturday, tens of thousands of protesters gathered for rival rallies on Crimea.
Supporters of the Crimea referendum waved Russian and Soviet Union flags as they marched to Moscow's Revolution Square. Many of them wore identical red and black outfits.
Separately, chanting opponents waved Ukrainian and Russian flags. Some voiced concern that Russian intervention in Crimea could lead to war.
Also, hundreds of people turned out for a rally in Kiev where they voiced support for Crimea remaining a part of Ukraine.
International monitors began arriving in Crimea on Saturday to observe Sunday's referendum vote. The monitors were invited by Russia and pro-Russian Crimean officials.
Alexander Simov, a Bulgarian journalist, said he believes the vote is legitimate.
"I am going to watch the whole process. To see... I think it that it is a legal referendum. So I think it is going to be very legal."
Another monitor, Helsinki University professor Johan Backman, said Western powers do not fully understand the situation in Crimea.
"Western politicians cannot understand what is going on. Because as far as I understand these countries and especially the U.S. have very weak informational base about what is going on in Russia first and secondly what is happening in Ukraine. And also western politicians of EU they do not really understand what happens in Russia. Western countries are at a dead end. In fact they cannot take any measures against Russia in this situation. And Russia is in control totally and securely."
U.S. Secretary of State John Kerry says Russian acceptance of a Crimean referendum to break off from Ukraine and join Russia would be an illegal "backdoor annexation."
Kerry met with Russian Foreign Minister Sergei Lavrov for six hours in London Friday in an effort to defuse the tensions in Crimea.
If Sunday's vote passes and the Russian parliament ratifies it, Kerry said that would violate international law, and there will be consequences. He said this is not a threat against Russia but a matter of respecting international standards for annexation and independence.
Lavrov said in a separate news conference Friday that the talks with Kerry were useful, but the two have "no common vision" on Crimea. He said Russia will "respect the will of the Crimean people," and he criticized the threat of U.S. and European Union sanctions on Russia as "counterproductive."
U.S. President Barack Obama said Friday he still hopes for a diplomatic solution to the crisis. But Kerry said it is clear Russian President Vladimir Putin will not make any moves until after Sunday's referendum.
The Kremlin says Mr. Putin told U.N. Secretary-General Ban Ki-moon in a phone call that the referendum is "fully consistent with international law and the U.N. Charter." However, the U.S. and European Union say the referendum violates Ukraine's constitution and international law.
Moscow acknowledged Thursday that it is deploying thousands more troops and military hardware near the Ukrainian border for two weeks of military maneuvers.
The U.S. State Department says it is "very concerned" about the deployment. The U.S. estimates that Russia already may have 20,000 troops in Crimea.
U.S. Vice President Joe Biden is traveling to Poland and Lithuania next week to meet with regional partners to discuss events in Ukraine. A White House statement says Biden will consult on steps to support Ukraine's sovereignty, and affirm international "collective defense commitments under the North Atlantic Treaty."