The referendum, widely expected to be passed by an overwhelming majority, is at the center of the biggest showdown between Russia and the West since the end of the Cold War.
Western and Ukrainian leaders have decried thereferendum as illegitimate, noting that masked troops that seized key military sites on the peninsula two weeks ago and widely believed to be under Russian command necessarily undermine the neutrality of the voting process.
But with international observers looking on, it appeared by Sunday evening that the referendum process, sure to remain the subject of prolonged controversy, was proceeding without major incident at the over 1,000 polling sites in the largely Russian-ethnic region.
By 4 p.m. local time, 70 percent of eligible voters had already cast ballots, according to voting officials. Polls were scheduled to close at 8 p.m.
"Currently everything we have seen was without violations or provocations, there is nothing that one could say would make the vote invalid,” said Ivan Abazher, an election observer from Bulgaria.
"The voting is ongoing in a systematic, organized fashion, with no serious violations,” Leonid Slutsky, the head of a Russian parliamentary delegation of observers said.
Berto Fabrizio, an Italian deputy of the European Parliament observing the referendum, said that he had witnessed no pressure exerted on voters.
Sunday’s referendum, with ballots in the Tatar, Russian and Ukrainian languages, asked voters if they prefer secession from Ukraine and annexation by Russia, or remaining as part of Ukraine but with expanded autonomy for the region.
There was no option for retaining the status quo of the region within Ukraine or for Crimea to become an independent republic without Russian annexation.
A senior Kremlin spokesman said that President Vladimir Putin was closely following the referendum while in Sochi to attend the closing ceremony of the Paralympic Games.