* 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 (Adds colour, fresh quotes, Russian troop numbers)
By Aleksandar Vasovic and Mike Collett-White
SIMFEROPOL, Ukraine, March 16 (Reuters) - Crimeans voted on Sunday on 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, turnout appeared to be high as people lined up to cast their ballots.
"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.
When Crimean Prime Minister Sergei Aksyonov, whose election is not recognised by Kiev, cast his ballot, a man tried to unfurl a blue and yellow Ukrainian flag next to him, but people in the crowd stopped him.
Voters have two options to choose from - but both imply Russian control of the peninsula.
"CURSE THEM ALL"
Ukraine's acting president Oleksander Turchinov late on Saturday called on people in Crimea to boycott the "pseudo-referendum".
He said: "Its result has already been written in the Kremlin, which needs some grounds to officially put troops on our land and start a war which will destroy people's lives and the economic prospects of Crimea."
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.
Shevkaye Assanova, a Crimean Tatar in her 40s, said she would not recognise the outcome of the vote.
"This is my land. This is the land of my ancestors. Who asked me if I want it or not? Who asked me? For the rest of my life I will be cursing those who brought these people here. I don't recognise this at all. I curse all of them."
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 an uprising in which more than 100 people were killed.
The protests began when Yanukovich turned his back on a trade agreement 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.
German Foreign Minister Frank-Walter Steinmeier said the situation was "combustible".
"We didn't ask for this confrontation. But if Russia doesn't come around at the last minute, us EU foreign ministers will give an appropriate first response on Monday," he told the Welt am Sonntag Sunday newspaper.
"So far Russia refuses every exit option, every step of de-escalation and apparently wants to create precedents we can't accept," he said.
Germany receives more than a third of its gas and oil from Russia and over 6,000 German firms are active there. Since the start of the crisis, Berlin emerged in a front seat in diplomacy with Russia but Chancellor Angela Merkel has now made clear that time is running out if Putin continues to snub diplomacy.
THOUSANDS OF SOLDIERS
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 where many Western reporters covering Sunday's referendum were 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.
Aksyonov 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.
But the Russian military has done little to hide the arrival of thousands of soldiers, along with trucks, armoured vehicles and artillery.
Ukraine's acting defence minister said there were now 22,000 Russian troops in Crimea, well beyond the 12,500 allowed under agreement covering its Black Sea Fleet based in Sevastopol.
Adding to tensions on the eve of the referendum, Ukraine's military confronted Russian forces which crossed Crimea's regional border on a 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 said he would protect ethnic Russians and Russian-speakers in Ukraine. (Additional reporting by Andrew Osborn in Simferopol and Ron Popeski in Kiev and Annika Breidthardt in Berlin,; Writing by Mike Collett-White; Editing by Richard Balmforth and Angus MacSwan)