* Russian troops in Crimea also double agreed level - Kiev
* Ukraine will not leave Crimea - minister
* Moscow rejects accusations that vote is illegal
* Tensions spread in cyberspace, NATO websites attacked
By Mike Collett-White and Ronald Popeski
SIMFEROPOL, Ukraine/KIEV, March 16 (Reuters) - Crimeans voted in a referendum on Sunday on whether to break away from Ukraine and join Russia, with Kiev accusing Moscow of rapidly building up its armed forces on the peninsula in "crude violation" of an international treaty.
Caught in an East-West crisis reminiscent of the Cold War, Ukrainian acting defence minister Ihor Tenyukh said Russian troop numbers in Crimea were now almost double the level agreed with Moscow, and Kiev's forces were taking "appropriate measures" along the border with Russia.
Tenyukh dismissed any suggestion that a militarily and economically weakened Ukraine might give up in the face of the Russian power.
"Decisions will be taken depending on how events unfold. But let me say once again that this is our land and we will not be leaving it," he told Interfax news agency.
Western countries say the vote, which is likely to favour union with Russia for a region which has a Russian-speaking majority, is illegal and being conducted at the barrel of a gun.
At the United Nations, 13 Security Council members voted for a draft resolution saying the result should not be recognised internationally, but Moscow exercised its veto while China abstained. Russian Foreign Minister Sergei Lavrov rejected the Western accusations, telling his U.S. counterpart John Kerry that the referendum complied with international law.
Both the West and Kiev have been powerless to stop the referendum. At a polling booth at a 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."
Last month's fall of Moscow-backed Ukrainian President Viktor Yanukovich following deadly protests in Kiev has aroused fears among some of the country's native Russian-speakers.
"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?" said Vasilyeva.
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 a day or two later.
Crimea's 1.5 million voters have two options: union with Russia or giving their region, which is controlled by pro-Kremlin politicians, the broad right to determine its own path and choose relations with whom it wants - including Moscow.
A CRUDE VIOLATION
Russia has the right to keep forces on the Black Sea peninsula, including at its naval base in the port of Sevastopol, under a treaty signed after Ukraine gained independence from the wreckage of the Soviet Union in 1991.
But Tenyukh accused Moscow of going far beyond an agreed limit on servicemen which he said was 12,500 for 2014. "Unfortunately, in a very short period of time, this 12,500 has grown to 22,000. This is a crude violation of the bilateral agreements and is proof that Russia has unlawfully brought its troops onto the territory of Crimea," he said.
This figure had risen from 18,400 on Friday. "The Ukrainian armed forces are therefore taking appropriate measures along the southern borders," he said.
Many Crimeans hope union with Russia will bring better pay and make them citizens of a country capable of asserting itself on the world stage. But others see the referendum as a land grab by the Kremlin from Ukraine, whose new rulers want to move the country towards the European Union and away from Russia's sway.
Ethnic Tatars, Sunni Muslims who make up 12 percent of Crimea's population, said they would boycott the vote despite promises by the authorities to give them financial aid and proper land rights.
"This is my land. This is the land of my ancestors. Who asked me if I want it or not? Who asked me?," said Shevkaye Assanova, a Crimean Tatar in her 40s. "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."
PROTECTION FROM "FASCISTS"
Russian President Vladimir Putin has justified his stance on Crimea by saying he must protect people from "fascists" in Kiev who ousted Yanukovich following the uprising in which more than 100 people were killed.
Western powers, preparing economic sanctions against Moscow over Crimea, largely dismiss his characterisation of the new authorities in Kiev as the successors of Nazi-allied Ukrainian forces which fought the Red Army in World War Two.
At the United Nations Russia vetoed on Saturday the draft resolution drawn up by the United States which called on "all states, international organizations and specialized agencies not to recognize any alteration of the status of Crimea on the basis of this referendum".
"This is a sad and remarkable moment," Samantha Power, the U.S. ambassador to the United Nations, said after the vote. "Crimea is part of Ukraine today. It will be part of Ukraine tomorrow. It will be part of Ukraine next week," she said.
Paris also tried to portray Moscow as isolated. "This annexation...goes beyond Ukraine, it concerns us all," Gerard Araud, France's U.N. ambassador, said in a statement. "This veto must be seen as a defeat only for Russia."
However, the Russian Foreign Ministry said on Sunday that Lavrov had told U.S. Secretary of State Kerry in a phone call the previous day that the referendum was legal.
"Lavrov reiterated that the Crimean referendum fully complies with international law and the United Nations Charter and the results should be the starting point in determining the future of the peninsula," the ministry said in a statement.
TENSIONS IN CYBERSPACE
Tenions over Crimea appear also to be spreading in cyberspace. Unidentified hackers brought down several public NATO websites with attacks on Saturday, the alliance said.
Spokeswoman Oana Lungescu said on Twitter that the attacks, which began on Saturday evening, continued on Sunday, although most services had now been restored.
"It doesn't impede our ability to command and control our forces. At no time was there any risk to our classified networks," another NATO official said.
A group calling itself "cyber berkut" - named after riot police formally disbanded by the central powers in Kiev - said the attack had been carried out by patriotic Ukrainians angry over what they saw as NATO interference in their country.
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 with 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. (Additional reporting by Andrew Osborn in Simferopol, Ron Popeski and Richard Balmforth in Kiev, Mirjam Donath at the United Nations, Adrian Croft in Brussels, Peter Apps in London; Writing by David Stamp)