Some 1,200 polling stations opened Sunday in Crimea for a referendum to decide whether the autonomous republic will break away from Ukraine, Xinhua reported.
Polling stations will remain open from 8 a.m. to 8 p.m. (0600 GMT to 1800 GMT).
Witnessing the referendum are 135 foreign observers from 23 countries and 1,240 local ones. The vote also attracted some 2,500 journalists from around the world.
The final results of the referendum will not be announced until Monday. But an exit poll has been commissioned by the Kryminform news service.
Crimea held referendums in 1991 and 1994. This year's referendum asks voters whether they wanted to reunify with Russia or they wanted to remain within Ukraine with the peninsula's 1992 constitution restored to give Crimea greater autonomy.
The autonomous republic's authorities predicted that voter turnout would be at least 80 percent.
On March 6, the Crimean parliament voted to become part of Russia and decided to hold a referendum Sunday on Crimea's future status.
The regional parliament on Tuesday adopted a declaration of independence from Ukraine, which specifies that Crimea will break away from Ukraine if around 2 million residents vote in favor of joining Russia in the referendum. But Ukraine's constitutional court has ruled the referendum unconstitutional.
On Saturday, Moscow vetoed a UN Security Council draft resolution, drawn up by the United States and backed by Western countries, that declares the referendum invalid. Russia will "respect the will of the Crimean people," said Vitaly Churkin, Russian ambassador to the United Nations.