Ukraine's southern Crimea region has voted in a referendum aimed at deciding whether the peninsula leaves Ukraine and becomes part of Russia.
Around 1.5 million registered voters are expected to have voted overwhelmingly in favour of breaking away from Ukraine after polls open at 06:00 GMT. Exit poll results will be announced 12 hours later, shortly after voting ends at 18:00 GMT.
Al Jazeera's Jacky Rowland, reporting from Simferopol, said the whole atmosphere leading up to the vote only pointed towards one outcome - reunification with Russia.
There are two options available in the referendum. Firstly, "are you in favour of the reunification of Crimea with Russia as a part of the Russian Federation?" The second option is "are you in favour of restoring the 1992 Constitution and the status of Crimea as a part of Ukraine?"
Returning to the 1992 constitution would give Crimea's government sweeping powers to make its own laws and control its own governance, while technically remaining part of Ukraine.
The West has said it will not recognise the referendum, while Moscow insists it is an example of self-determination like Kosovo.
Russian troops and pro-Moscow armed men took control of the strategic peninsula with a majority ethnic-Russian
population soon after the Kremlin-backed Ukrainian President Viktor Yanukovich fled Kiev last month after three months of deadly protests against his rule.
Ihor Tenyukh, Ukraine's acting defence minister, said that the defence ministries of Ukraine and Russia have agreed on a truce in Crimea until March 21, in which "no measures will be taken against our military facilities".
"Our military sites are therefore proceeding with a replenishment of reserves," he added, just moments after saying that his government has no intention of pulling its troops out of Crimea, despite the rapid build-up of Russian troops in the region to a level of about 22,000 servicemen.
The referendum comes a day after Russia was exposed as isolated over the crisis in Ukraine, with members of the UN Security Council overwhelmingly supporting a draft resolution condemning the vote on the future of Crimea as illegal.
Despite this, our correspondent in Simferopol said the referendum had a strong pro-Russia atmosphere.
She said even the music playing in the background of polling stations were Soviet songs that evoked memories of the "good old days".
Meanwhile, Russian troops landed on a strip of land in the southeast between Crimea and the mainland, forcing Ukraine to dispatch an aircraft and armed forces to stop the troop movement.
Russia vetoed the US-backed resolution when it was put to a vote before the council, but its ally China abstained, leaving it as the only nation to recognise the Crimean referendum.
The veto means the resolution will not be adopted by the UN.
After the vote was taken, Samantha Power, the US ambassador to the UN, said: "Under the UN charter, the Russian Federation has the power to veto a Security Council resolution. But it does not have the power to veto the truth.
"History has lessons for all of us, who are willing to listen. Unfortunately, not everyone was willing to listen today."
Al Jazeera's diplomatic editor, James Bays, said the vote laid bare the international opposition to Russia's stance on Ukraine and Crimea.
"China is not supporting its ally Russia on this occasion. It is abstaining. That's the best the Western nations, who drafted this resolution, could hope for, but they think that this is important because it exposes that Russia is on its own."
Before the vote, Russia's ambassador to the UN, Vitaly Churkin, said that Russia's action in Crimea arose because of an "unconstitutional coup" in Kiev.
Meanwhile, Ukraine accused Russia of further incursion into its territory, which the US said would be an "outrageous escalation" if proven to be true.
"For something additional, even more escalatory to have occurred would be flying in the face of everything you have heard here today," Ambassador Power said.
In Moscow, about 50,000 people rallied to protest against Russia's intervention in Ukraine, shouting, "The occupation of Crimea is Russia's disgrace" and "Hands off Ukraine".