Crimea's pro-Russian authorities say voters in that region of Ukraine have overwhelmingly backed union with Russia in a referendum denounced by Ukraine's government and the West as illegal.
Local election officials said that with about three-quarters of the ballots counted, more than 95 percent of voters support leaving Ukraine and joining Russia.
Exit polling commissioned by authorities and announced immediately after polls closed suggested 93 percent of voters in favor of being part of Russia.
Officials said turnout was over 81 percent.
Crimea's pro-Russian prime minister has said a Crimean parliament delegation will travel to Moscow on March 17 to discuss the region's accession to Russia.
Many among Crimea’s ethnic Tatar population and pro-Kyiv Ukrainians were expected to boycott the vote.
Russian forces since late last month have been occupying Crimea, where there are currently some 22,000 troops, according to Ukraine's acting defense minister. Thousands more pro-Russian "self-defense" troops have been mobilized, including by the republic's breakaway authorities.
The United States and the European Union have rejected the referendum, which they say is against Ukraine's constitution and international law.
U.S. President Barack Obama told Russian President Vladimir Putin the referendum will never be recognized by the international community.
According to the White House, Obama also said the United States and its European partners were prepared to "impose additional costs" on Russia for violating Ukraine's sovereignty.
According to the Kremlin, Putin told Obama the referendum was legitimate and he expressed concern over what he said was the Ukrainian government's failure to stamp out violence against Russian speakers in the country.
Top EU officials called the referendum "illegal and illegitimate" and said EU foreign ministers will decide on "additional measures" against Moscow on March 17.
The British and the French foreign ministers, in separate statements, also rejected the referendum.
In the referendum, voters were asked whether Crimea -- where ethnic Russians are about 60 percent of the population -- should break away from Ukraine and become part of Russia, or opt for greater autonomy.
The head of the election commission in Crimea, Mikhail Malyshev, said voting proceeded without incident.
"[The electoral] commission has not received any complains -- neither regional commissions nor the central one -- regarding the procedure of the referendum," Malyshev said. "Momentarily, commissions at individual polling stations have started ballot count. I want to inform you all that the Crimean referendum is considered valid."
Ukraine's government has called the vote a "circus" directed at gunpoint by Moscow.
Provisional results were expected to be released late on March 16.
Ukrainian Prime Minister Arseniy Yatsenyuk vowed to bring to justice all those promoting separatism and undermining the Ukrainian constitution, individuals he suggested were "protected" by Russian troops.
"Together with our Western partners we will do everything possible to make sure that everyone, who today feels protected enough by Russian guns to do whatever they please, knows that they will have to answer for separatism and attempts to destroy the constitutional order," Yatsenyuk said. "No place in the world will be safe enough for them to disregard the law. And Russia will not save them."
Pro-Russian Unrest Mounts In Eastern Ukraine
Pro-Moscow demonstrators rallied in eastern and southern Ukrainian cities as the voters in Crimea cast ballots.
In Donetsk, some 5,000 protesters marched through the city, smashing doors and windows as they went.
Chanting "Donetsk, Crimea, Russia," they also called for their own referendum on secession from Ukraine.
The demonstrators massed outside the prosecutor’s office in Donetsk and demanded the release of the self-declared pro-Russian governor, Pavel Gubarev, who was detained earlier this month on charges of calling for the overthrow of Ukraine's constitutional authorities.
They briefly occupied the building, tearing down a Ukrainian flag and replacing it with a Russian flag.
WATCH: RFE/RL Ukrainian Service video of the pro-Russia protest in Donetsk and the storming of the regional prosecutor's offices on March 16:
The Donetsk crowd also attacked the regionial headquarters of Ukraine’s SBU intelligence service for the second straight day.
Donetsk is a former stronghold of Ukraine’s ousted President Viktor Yanukovych.
Farther to the north, in Kharkiv, about 6,000 pro-Russia demonstrators defied a protest ban and held their own symbolic referendum and rally for more autonomy and “sovereignty” for Russian-language speakers.
Rally organizers handed out improvised ballots that protesters could "cast" in plastic bags to ask for enhanced autonomy.
At one point, a Ukrainian judicial official who came to inform the protesters of the demonstration ban was chased off by the jeering crowd, which chanted "Russia!" "Referendum!" and "Crimea, we are with you!"
Kharkiv’s pro-Russian activists also broke into a Ukrainian cultural center, removing Ukrainian-language books and setting them alight in the street in a series of small bonfires.
Several thousand pro-Russian demonstrators also marched through the streets of the Black Sea port of Odesa.
Two cities -- Luhansk in the northeast and Mikolayiv in the south -- also held impromptu "referendums," banned by local courts, that echoed the vote in Crimea.
Based on reporting by Reuters, RFE/RL's Ukrainian Service, AP, AFP, and Interfax