* Crimean referendum seen backing union with Russia
* Security tight across Black Sea peninsula
* Russian troops in control, despite official denials
* Vote set to trigger sanctions in major East-West crisis
Crimeans decided on Sunday whether to break away from Ukraine and join Russia in a referendum that has alarmed the ex-Soviet republic and triggered the worst crisis in East-West relations since the Cold War.
Thousands of Russian troops have taken control of the Black Sea peninsula, and Crimea's pro-Russian leaders have sought to ensure the vote is tilted in Moscow's favour.
That, along with an ethnic Russian majority, is expected to result in a comfortable "Yes" vote to leave Ukraine, a move that could prompt U.S. and European sanctions as early as Monday against those seen as responsible for the takeover of Crimea.
Polling stations opened at 8 a.m. (0600 GMT) and close 12 hours later. Provisional results will be released late on Sunday with the final tally expected one or two days later.
At a polling booth inside a high school in Simferopol, the Crimean regional capital, dozens of people lined up outside to cast their ballots early.
"I have voted for Russia," said Svetlana Vasilyeva, a veterinary nurse who is 27. "This is what we have been waiting for. We are one family and we want to live with our brothers.
"We want to leave Ukraine because Ukrainians told us that we are people of a lower kind. How can you stay in such a country?"
The majority of Crimea's 1.5 million electorate, like Svetlana, support leaving Ukraine and becoming part of Russia, citing expectations of better pay and the prospect of joining a country capable of asserting itself on the world stage.
But others see the referendum as nothing more than a geo-political land grab by the Kremlin which is seeking to exploit Ukraine's relative economic and military weakness as it moves towards the European mainstream away from Russia.
Ethnic Tatars, Sunni Muslims of Turkic origin who make up 12 percent of Crimea's population, said they would boycott the referendum, despite promises by the authorities to give them financial aid and proper land rights.
Russian President Vladimir Putin has justified his stance on Crimea by saying he must protect people from "fascists" in Kiev who ousted the Moscow-backed Viktor Yanukovich in February following a violent uprising in which more than 100 people were killed.
The protests began when Yanukovich turned his back on a trade deal with the European Union and opted for a credit and cheap oil deal worth billions of dollars with Ukraine's former Soviet overlord, Russia.
Kiev and Western governments have declared the referendum illegal, but have been powerless to stop it.
Voters have two options to choose from - but both imply Russian control of the peninsula.
On the surface, the second choice appears to offer the prospect of Crimea remaining with Ukraine.
However, the 1992 constitution which it cites foresees giving the region effective independence within Ukraine, but with the right to determine its own path and choose relations with whom it wants - including with Russia.
The streets of Simferopol have been largely calm in the days leading up to the vote, although the heavy presence of armed men, many wearing black balaclavas, has created an unnerving atmosphere in the normally sleepy town.
On Saturday night, about 30 men in balaclavas carrying automatic weapons barged into the Hotel Moscow, a Soviet-era hotel where many Western reporters covering Sunday's referendum are staying.
They said they had come to investigate an unspecified security alert and did not threaten anyone, but some witnesses saw it as a move to intimidate journalists.
Crimean Prime Minister Sergei Aksyonov, whose election two weeks ago in a closed session of the regional parliament is not recognised by Kiev, does not officially acknowledge that Russian troops are in control of Crimea - a position also maintained by Moscow.
They say that thousands of unidentified armed men, visible across the region, belong to "self-defence" groups created to ensure stability.
THOUSANDS OF SOLDIERS
But the Russian military, which leases the Crimean naval base of Sevastopol from Ukraine, has done little to hide the arrival of thousands of soldiers, along with trucks, armoured vehicles and artillery.
Masked gunmen surrounding Ukrainian military bases in Crimea have identified themselves as Russian troops.
Adding to tensions on the eve of the referendum, Ukraine's military confronted Russian forces which crossed Crimea's regional border on a remote sand spit, some 30 km (20 miles) off the mainland.
Crimea's separatist government said its own forces had moved to defend a gas pumping station. Ukrainian officials said no shots were exchanged.
What happens to Ukrainian forces in Crimea after Sunday's vote is one of many unanswered questions. Commanders are nervous about how Russia will go about taking control of military bases where Ukrainian forces are still armed.
Crimean authorities have said Ukrainian servicemen will have the choice of surrendering their weapons and walking away peacefully or joining pro-Russian local forces.
In the run-up to the referendum, the worst violence in Ukraine has been in the east, where acting president Oleksander Turchinov said there had been three deaths in two days.
He also said there was "a real danger" of invasion by Russian troops across the eastern border. The area has a large number of Russian-speakers - significant since Putin has vowed to protect ethnic Russians and Russian-speakers in Ukraine.