Athens - Greek Prime Minister Alexis Tsipras said he's "optimistic" about the outcome of the country's bailout referendum as he casts his ballot at a polling station in the Kypseli neighbourhood of central Athens.
"Today is a day of celebration because democracy is a celebration," he said.
"Many may ignore the will of a government but not the will of the people to live, to live with determination, to take life into their own hands," he said.
"I am confident that tomorrow we will open a way for all the people of Europe," said the hard-left prime minister who urged supporters to vote "no".
Greeks were casting ballots on Sunday in a snap referendum on the country's bailout conditions, in a vote that has profound consequences for the debt-laden country and the rest of Europe.
A call for unity
Greek President Prokopis Pavlopoulos called for unity whatever the result, saying "we move on, all together" as he cast his ballot in the capital.
The referendum asks Greeks whether they approve of the bailout conditions offered before Athens stopped talks with the country's creditors - the European Commission, the European Central Bank and the International Monetary Fund (IMF).
A series of opinion polls showed a tight race, with a slim lead for the "yes" camp that is well within the margin of error. The polls, released on Friday, also showed an overwhelming majority of people want Greece to remain in the eurozone.
The nationwide ballot was taking place at the end of a week that saw Greece close its banks, cap cash withdrawals, implement capital controls, fail to repay a loan to the IMF and lose billions of euros when its bailout programme expired.
Ejected from the eurozone
Fears are rife that Greece could become the first nation to be ejected from the eurozone, and the referendum is widely perceived as a vote on whether the country should remain in the bloc or not.
"It's all terrible," said a pensioner outside a polling station in the Thissio neighbourhood of Athens. "For them to cut my pension? No!" another said. The two elderly ladies declined to give their names.
Vasilios Mantelas, 65, said he deliberately cast an invalid vote. "I want Greece out of the European Union and out of memorandums," he said, referring to the nation's bailout programmes.
A vote of confidence in Tsipras' government
The referendum also serves as a nationwide vote of confidence in Tsipras' government, which came to power in January on an anti-austerity ticket. Analysts say he will have no choice but to resign if the "yes" vote wins.
Tsipras has repeatedly said he wants Greece to remain in the eurozone and told voters that rejecting the creditors' proposals will give him a mandate to negotiate a better deal. But European officials have warned that it will be much harder to reach agreement on any new bailout arrangements if the country votes "no".
Some 9.8 million Greeks were eligible to vote in the referendum at 19.159 polling stations across the country.
Polls opened at 04:00 GMT and are expected to close at 16:00 GMT. There will be no exit polls due to cost constraints.
The first reliable estimate of the results could come as early as 18:00 GMT provided that there is a clear 4% difference between "yes" and "no", and that 10% of the ballots from the entire country have been counted.