By Nick Martin, News Correspondent in Simferopol
The Crimean people are voting in a referendum widely expected to transfer control of the Black Sea region from Ukraine to Moscow, despite an outcry and threat of sanctions from the West.
The vote, dismissed by Kiev and Western governments as illegal, has triggered the worst East-West crisis since the Cold War and marks a new peak in turmoil in Ukraine.
The Crimean authorities said there had been a turnout of 80%.
Russian President Vladimir Putin has reiterated the referendum complies with international law and promised to "respect" the outcome of the vote on whether to join Russia.
Election commission officials count ballots ahead of the referendumBut the build up of Russian forces in the region has fuelled claims the poll is being conducted "at the barrel of a gun", although Crimea's new pro-Moscow Prime Minister Sergei Aksyonov insisted the vote would be "open and transparent".
As people were casting their ballots the Ukrainian and Russian defence ministries announced a truce in the region until March 21.
Cossacks and pro-Moscow militias were seen patrolling some polling stations and Russian flags were seen flying across the area. No violence has been reported so far.
Voting will end at 6pm UK time and early results are expected later on Sunday evening.
More than 20,000 Russian troops are now stationed in Crimea, troop movements Ukraine has called an invasion.
The country's interim Prime Minister Arseniy Yatsenyuk, announced a call-up to try and raise 20,000 men, vowing to bring to justice those he said were trying to destroy Ukrainian independence "under the cover of Russian troops".
Polling booths are readied for the crucial breakaway voteThe unrest follows events in November when the now ousted President Viktor Yanukovych walked out on a trade deal with the European Union in favour of closer ties with Russia, sparking violent protests in Kiev.
The vote has split even the closest-knit families, many of whom say they want their peninsula to be governed in different ways. Elena Kruglova, 26, said Crimea should remain part of Ukraine, while her mother Lyna Losyeva is staunchly pro-Russian.
She said: "Two weeks to organise a referendum doesn't give people the chance to make a proper decision.
"At the moment, the way the referendum works there are two choices, Russia, or Russia. We are not being given the option to stay the way we are."
But her mother, who remembers being part of the former Soviet Union until Ukraine gained independence in 1991, disagrees.
She said: "I was born in a time when there was no difference between Russia and Ukraine and in the Soviet Union we didn't feel any differences.
"But my daughter was born in a different time.
Russia insists the vote is legal"What do I expect from Russia? I expect that Russia will listen to us, to Crimean people because for 23 years Ukraine didn't listen to us."
As voting was taking place, protests broke out against the redeployment of Ukrainian troops and armoured vehicles in the eastern cities of Kharkiv, Donetsk and Luhansk, according to eyewitnesses.
Thousands of pro-Russian protesters in Donetsk expressed support for the vote and pushed for their own referendum. The security headquarters and the prosecutor's office there have been stormed, according to reports.
Despite Russian's Foreign Minister Sergei Lavrov and US Secretary of State John Kerry talking daily since the crisis began, a diplomatic solution has not been reached.
In a phone call after voting got under way, the pair agreed to work on constitutional reform in Ukraine as a way of solving the crisis.
Johannes Anderson, an expert in Crimean affairs, believes Russia has a "grand plan" for Crimea. He said: "I think there's been a long-time dream for Russia to reincorporate Crimea into the Greater Russian empire.
"This is a broader trend of Russia pushing its imperial ambitions.
"Ukraine has been growing and emerging as an economy in recent years and this is Russia attempting to destabilise that growth and stamp its authority on the region."