The U.N. Security Council will vote Saturday on a U.S.-drafted resolution urging countries not to recognize the results of an upcoming Crimean referendum.
Sunday's referendum gives residents of Crimea only two choices: joining Russia or the significant strengthening of their autonomy within Ukraine. A vote in favor of joining Russia is widely seen as a forgone conclusion in the majority Russian-speaking region on the Black Sea, which has hosted czarist and Kremlin navies since the 18th century.
Diplomats say they expect Russia to veto Saturday's vote at the U.N., while supporters say 13 of the 15 Council members will approve the resolution. Meanwhile, China has not said how it will vote. If Moscow's close ally abstains, it would demonstrate Russia's extensive international isolation.
The U.N. resolution reaffirms Ukraine's territorial integrity, and declares Sunday's referendum "can have no validity."
On Saturday, tens of thousands of protesters gathered in central Moscow 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.
International monitors began arriving in Crimea on Saturday to observe Sunday's 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 power 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 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."