The vote was widely condemned by Western leaders, who planned to move swiftly to punish Russia with economic sanctions.
As the votes were counted, a jubilant crowd gathered around a statue of Vladimir Lenin in the center of Simferopol to celebrate with song and dance. Many held Russian flags, and some unfurled a handwritten banner reading "We're Russian and proud of it." Fireworks exploded in the skies above.
"We want to go back home, and today we are going back home," said Viktoria Chernyshova, a 38-year-old businesswoman. "We needed to save ourselves from those unprincipled clowns who have taken power in Kiev."
Ukraine's new government in Kiev called the referendum a "circus" directed at gunpoint by Moscow, referring to the thousands of troops that now occupy the peninsula, which has traded hands repeatedly since ancient times.
The referendum offered voters the choice of seeking annexation by Russia or remaining in Ukraine with greater autonomy. After 50 percent of the ballots were counted, more than 95 percent of voters had approved splitting off and joining Russia, according to Mikhail Malishev, head of the referendum committee.
Final results were not expected until Monday.
Opponents of secession appeared to have stayed away Sunday, denouncing the vote as a cynical power play and land grab by Russia.
Putin insisted the referendum was conducted in "full accordance with international law and the U.N. charter."
Russia was expected to face strong sanctions Monday from the U.S. and Europe for going forward with the vote, which could also encourage rising pro-Russian sentiment in Ukraine's east and lead to further divisions in this nation of 46 million. Residents in western Ukraine and the capital, Kiev, are strongly pro-West and Ukrainian nationalist.
Andrew Weiss, vice president for Russian and East European studies at the Carnegie Endowment for International Peace, suggested the confrontation could intensify.
Russia "is really turning its back on the outside world and is basically going to say to the West, 'Now, go ahead. Show us how tough you are.' And the West, I think, is struggling to come with an adequate response."
The Crimean parliament planned to meet Monday to formally ask Moscow to be annexed, and Crimean lawmakers were to fly to Moscow later in the day for talks, Crimea's pro-Russia prime minister said on Twitter.
Russian lawmaker Vladimir Zhirinovsky said the annexation could take "from three days to three months," according to the Interfax news agency.
Some residents in Crimea said they feared the new Ukrainian government that took over when President Viktor Yanukovych fled to Russia last month would oppress them.
"It's like they're crazy Texans in western Ukraine. Imagine if the Texans suddenly took over power (in Washington) and told everyone they should speak Texan," said Ilya Khlebanov, a voter in Simferopol.
Ukraine's new prime minister insisted that neither Ukraine nor the West would recognize the vote.
"Under the stage direction of the Russian Federation, a circus performance is underway: the so-called referendum," Prime Minister Arseniy Yatsenyuk said Sunday. "Also taking part in the performance are 21,000 Russian troops, who with their guns are trying to prove the legality of the referendum."
As soon as the polls closed, the White House again denounced the vote.
"The international community will not recognize the results of a poll administered under threats of violence," it said in a statement. "Russia's actions are dangerous and destabilizing."
Russia raised the stakes Saturday when its forces, backed by helicopter gunships and armored vehicles, took control of the Ukrainian village of Strilkove and a key natural gas distribution plant nearby - the first Russian military move into Ukraine beyond the Crimean peninsula of 2 million people.
The Russian forces later returned the village but kept control of the gas plant. On Sunday, Ukrainian soldiers were digging trenches and erecting barricades between the village and the gas plant.
"We will not let them advance further into Ukrainian territory," said Serhiy Kuz, commander of a Ukrainian paratrooper battalion.
Despite the threat of sanctions, Putin has vigorously resisted calls to pull back in Crimea. At the United Nations on Saturday, Russia vetoed a Security Council resolution declaring the referendum illegal.
However, Putin spoke with President Barack Obama and supported a proposal from Germany to expand an international observer mission in Ukraine, the Kremlin said Sunday in a statement after the vote.
"The heads of state noted that despite the differences in their assessments, it was necessary to work together to find a way to stabilize the situation in Ukraine," the statement said.
German Chancellor Angela Merkel, who also spoke with Putin on Sunday, wants more observers sent to tense areas, particularly in eastern Ukraine, her spokesman said. Putin told Obama that such a mission would be welcome but would need to be extended to all regions in Ukraine, the Kremlin said.
In Donetsk, one of the main cities in eastern Ukraine, pro-Russia demonstrators called Sunday for a referendum similar to the one in Crimea.
In Sevastopol, the Crimean port where Russia now leases a major naval base from Ukraine for $98 million a year, speakers blared the city anthem up and down the streets, but the military threat was not far away - a Russian naval warship still blocked the port's outlet to the Black Sea, trapping Ukrainian boats.
At a polling station inside a historic school, tears came to Vladimir Lozovoy, a 75-year-old retired Soviet naval officer, as he talked about his vote.
"I want to cry. I have finally returned to my motherland. It is an incredible feeling. This is the thing I have been waiting for for 23 years," he said.
But Crimea's large Muslim Tatar minority - whose families had been forcibly removed from their homeland and sent to Central Asia during Soviet times - remained defiant.
The Crimea referendum "is a clown show, a circus," Tatar activist Refat Chubarov said on Crimea's Tatar television station. "This is a tragedy, an illegitimate government with armed forces from another country."
The fate of Ukrainian soldiers trapped in their Crimean bases by pro-Russian forces was still uncertain. But Ukraine's acting defense minister, Igor Tenyuk, was quoted as saying Sunday that an agreement had been reached with Russia not to block Ukrainian soldiers in Crimea through Friday. It was not clear exactly what that meant.
Ethnic Ukrainians interviewed outside the Ukrainian Orthodox cathedral of Vladimir and Olga said they refused to take part in the referendum, calling it an illegal charade stage-managed by Moscow. Some said they were scared of the potential for widespread harassment.
Vasyl Ovcharuk, a retired gas pipe layer, predicted dark days ahead for Crimea.
"This will end up in military action, in which peaceful people will suffer. And that means everybody," he said. "Shells and bullets are blind."
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