According to results of an exit poll announced first on Russian media, 95% of voters backed a union with Moscow, 60 years after Soviet leader Nikita Khrushchev, an ethnic Ukrainian, gifted Crimea to the Ukrainian Soviet Socialist Republic on an apparent whim.
The outcome of Sunday's vote was never in doubt, although the main organiser said the figure was premature and that preliminary results would be announced later.
Thousands of people filled Lenin Square in the centre of Simferopol, Crimea's capital, and waved Crimean and Russian flags in a festive celebration of what most locals wanted.
Read: All you need to know about Crimea's referendum
A pro-Russian protester celebrates in Simferopol's Lenin Square on March 16, 2014 after exit polls showed that about 95% of voters in Ukraine's Crimea region supported union with Russia (AFP photo)
"We cannot be any worse off than we are now," said Lyudmila Sergeyevna, a 64-year-old who was born in Simferopol and has lived on the peninsula all her life.
"I am Ukrainian through and through, but I voted for Russia. I have a son, daughter and two grandsons living with me in a small apartment. I just hope things are going to be better now."
The majority of Crimea's 1.5 million electorate, like Sergeyevna, support becoming part of Russia, citing expectations of economic growth and the prospect of joining a country capable of asserting itself on the world stage.
But others saw 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 and away from Russia.
Thousands of Russian troops have taken control of the Black Sea peninsula, and Crimea's pro-Russian leaders ensured the vote was tilted in Moscow's favour.
That, along with an ethnic Russian majority, resulted in a comfortable "Yes" vote to leave Ukraine, a move expected to lead to U.S. and European sanctions as early as Monday against those seen as responsible for the takeover of Crimea.
When Crimean Prime Minister Sergei Aksyonov, whose election is not recognised by Ukrainian authorities in Kiev, cast his ballot, a man tried to unfurl a blue and yellow Ukrainian flag next to him, but people in the crowd prevented the show of dissent.
Voters had two options to choose from - but both implied Russian control of the peninsula.
Read: Ukraine or Russia? Crimeans set to vote
"Curse them all"
Ukraine's acting president Oleksander Turchinov late on Saturday called on people in Crimea to boycott the "pseudo-referendum", yet with two hours of polling still to go, turnout was officially 73.4%.
Turchinov 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."
Most ethnic Tatars, Sunni Muslims of Turkic origin who make up 12% of Crimea's population, boycotted 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.
"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 declared the referendum illegal, but were 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, referring to sanctions.
Read: Crimea holds secession referendum amid turmoil
Pro-Russian protesters celebrate in Simferopol's Lenin Square on March 16, 2014 after exit polls showed that about 95% of voters in Ukraine's Crimea region supported union with Russia (AFP photo)
Crimea's regional government will make a formal application today to join the Russian Federation, a local pro-Moscow regional leader said on Twitter after a disputed referendum to break away from Ukraine.
"The Supreme Soviet of Crimea will make an official application for the republic to join the Russian Federation at a meeting on March 17," Sergiy Aksyonov said in a tweet.
Thousands of Crimeans celebrated on Sunday the outcome of a referendum that exit polls said overwhelmingly backed the Black Sea peninsula's split from Ukraine and attachment to Russia, AFP reporters witnessed.
Several thousands gathered in the Crimean capital of Simferopol, waving Russian and Crimean flags, as well as in Sevastopol, home to Russia's Black Sea Fleet, where chants of "Russia!" accompanied an alcohol-fuelled celebration.
Read: Key events in Ukraine's political crisis
Thousands of soldiers
The streets of Simferopol were largely calm in the days leading up to the vote, although the heavy presence of armed men, many wearing black balaclavas, 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.
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 was in the east, where 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.
Read: All you need to know about Ukraine
(With inputs from AFP)