SIMFEROPOL, Ukraine — With thousands of armed Russian troops occupying the Crimean peninsula, a majority of Crimeans voted Sunday to secede from Ukraine and join Russia, carrying out a public referendum that Western leaders had declared illegal and vowed to punish with economic sanctions.
The outcome, in a region that shares a language and centuries of history with Russia, was a foregone conclusion even before exit polls showed more than 93 percent of voters opting to break with Ukraine and join Russia.
U.S. and European officials spent the day weighing how swiftly and forcefully to levy threatened sanctions against allies of Russian President Vladimir Putin.
President Obama told Mr. Putin in a phone call on Sunday that the United States rejected the results of the referendum and warned that Washington was ready to impose sanctions on Moscow.
He told the Russian leader that the crisis could still be resolved diplomatically, but said the Russian military would need to stop its “incursions” into Ukraine, the White House said.
The Crimean vote violates the Ukrainian constitution and occurred under duress of Russian military intervention, the President told Mr. Putin.
Mr. Obama asked Mr. Putin to support the deployment of international monitors to help prevent violence in Ukraine as that country prepares for spring elections.
“No decisions should be made about the future of Ukraine without the Ukrainian government,” the White House said, noting that Russia had rejected the deployment of international monitors in Crimea to ensure the rights of ethnic Russians there were protected.
“Russia’s actions are dangerous and destabilizing,” the White House said.
U.S. officials reaffirmed that the Obama Administration will, along with the European Union, impose penalties on Russia if it annexes the strategic region.
They also warned that any Russian moves on east and south Ukraine would be a grave escalation requiring additional responses.
With the voting in Crimea complete, attention shifted to Mr. Putin, who had stalled on the question of annexation by saying he wanted to hear the Crimean public proclaim its will.
Should he annex Crimea, Mr. Putin could find himself forced into negotiations with the fledgling government in Kiev that he has so far refused to recognize or meet, or face a serious conflict over water, energy, and other essentials for which Crimea is largely dependent on mainland Ukraine.
Mr. Putin also needs to decide what to do about Ukrainian military personnel, many surrounded for more than two weeks on bases throughout Crimea and refusing to surrender.
Unrest continued to swirl in eastern Ukraine as well. Russian troops have massed along the border, raising fears of a military incursion into mainland Ukraine.
In Kharkiv, several thousand pro-Russia demonstrators scuffled Sunday with police outside the governor’s office.
The crowd shouted, “Putin! Putin! Putin!” and “Crimea, we are with you!”
After pushing against Ukrainian police guarding the governor’s office for several minutes, the crowd marched to the Russian consulate, carrying Russian flags and freshly made red banners that read, “Russian Spring.”
Secretary of State John Kerry expressed concern in a phone call Sunday with Russian Foreign Minister Sergey Lavrov about “continuing provocations” in eastern Ukrainian cities, where U.S. officials have accused the Kremlin and its intelligence agents of fomenting unrest.
Mr. Kerry and Mr. Lavrov spoke by phone for the second straight day, following a six-hour meeting Friday. But they could not report any progress in defusing the crisis other than an agreement to continue working toward a political resolution.
Reflecting the huge gulf in perceptions that persists between the two sides of the divide, Mr. Putin told German Chancellor Angela Merkel in a phone call Sunday that the referendum was conducted in “full compliance with international law,” according to the Kremlin.
The United States and the European nations have denounced the referendum as illegal under international law and unconstitutional under Ukrainian law.
Putin spokesman Dmitry Peskov brushed aside a warning from the leaders of the Group of 7 world powers of unspecified consequences for Russia’s violation of international law in Crimea, saying it would have no effect on Russia’s policies.
Despite the uncertainties still surrounding Crimea, victory parties broke out long before the polls closed in Simferopol, the capital, and in Sevastopol, where Russia has long maintained the headquarters of its Black Sea fleet.
As they left the polls, after casting paper ballots, many voters were ebullient and expressed no concern about the soldiers with automatic weapons deployed across the peninsula.
Citizens with misgivings about joining Mr. Putin’s Russian Federation, particularly Crimean Tatars, a Muslim Turkic people with a history of persecution by Russia, generally opted to stay home rather than participate in what they called a rigged vote.
In Kiev, the government held an emergency cabinet meeting to discuss the continuing military threat in eastern Ukraine.
Acting Prime Minister Arseny Yatsenyuk told ministers that the government would prosecute the organizers of the referendum and others supporting Crimean separatism.
Mr. Yatsenyuk said the organizers were now “under the cover of Russian troops” but that the Ukrainian government would “bring them to justice in Ukrainian and international courts.”
Crimea was effectively part of Russia from the late 1700s until the dissolution of the Soviet Union in 1991.
Geographically, however, the peninsula is isolated from Russia.
Annexing it could prove complicated.