Millions of Greeks have voted in a crucial referendum on whether to accept the terms of an international bailout.
Opinion polls released as voting ended suggested a slight lead for the "No" vote.
No exit polls were published. The first official results are expected in the coming hours.
The government had urged people to vote "No", while the "Yes" campaign warned that this could see Greece ejected from the eurozone.
The governing radical-left Syriza party has criticised the bailout terms as humiliating. Its leading figures say rejecting the terms could give them more leverage in talks over the country's massive debt.
Prime Minister Alexis Tsipras said on Sunday that a "No" would show Greece's determination "not only to stay in Europe but to live with dignity in Europe".
However, international creditors have warned that a "No" vote could choke off vital funding for Greek banks and lead to "Grexit" - a chaotic departure from the common European currency.
The "Yes" campaign has framed the vote as a referendum on Greek membership of the eurozone.
At the scene: Chris Morris, BBC News, Athens
Voting has ended and a nation is holding its breath. The rest of the eurozone is watching just as closely - can a government from the radical left buck the European trend? Much will depend on how the numbers come in over the next few hours.
Greek Prime Minister Alexis Tsipras's tone throughout the past week has been defiant - suggesting that a "No" vote would allow him to return to negotiations with a stronger hand. But there are many people in Greece who fear that he could be gambling with their future.
It looks like the vote will be close, and that might suggest the nation is divided. But a clear majority of Greeks say they want to stay in the eurozone. The risk is that the tumultuous events of the past few weeks have made that much more difficult.
By the afternoon, local media were reporting a high turnout.
"In the end I went for 'Yes'," one voter in Athens, Maya Korba, told the BBC. "I don't really know what both answers will mean and to be honest I'm not happy with either, but we have to hope."
Banks have been shut and capital controls in place since the start of the week, after the European Central Bank declined to give Greece more emergency funding.
Withdrawals at cash machines have been limited to €60 per day.
The country's current bailout expired on Tuesday and Greece missed a €1.6bn (£1.1bn) payment to the International Monetary Fund (IMF).
Robert Peston, BBC economics editor, Athens
Since I arrived in Athens, I have witnessed Greeks queuing at those cash machines that are working, to withdraw the maximum amount of cash they're allowed under the restrictions implemented last Monday.
"My concern is that if there is no easing up of the restrictions, companies will start laying off workers tomorrow," said a senior banker.
He added that the so-called capital controls, which include a ban on movement of money abroad, are leading to chronic shortages of medicines, foods, vital raw materials and other important goods.
Casting his ballot, Finance Minister Yanis Varoufakis described the referendum as "a holy moment" that "gives hope that the common currency and democracy can co-exist".
Mr Varoufakis earlier told local media that the EU had "no legal grounds" to throw Greece out of the euro.
The finance minister said banks would reopen on Tuesday, whatever the outcome, and that Mr Tsipras would still reach an agreement with creditors if the result was "No" in the referendum.
French Economy Minister Emmanuel Macron said negotiations should continue even in the case of a win for the "No" vote, and that Europe could not "crush an entire people".
Meanwhile, German Finance Minister Wolfgang Schaeuble, one of Greece's harshest critics, suggested that if Greece were to leave the eurozone, it might only be temporary.
Greece was a member of the eurozone, "whether with the euro or temporarily without it - only the Greeks can answer this question," he told the German newspaper Bild on Saturday. "And it is clear that we will not leave the people in the lurch."
Ballot paper question
"Should the agreement plan submitted by the European Commission, the European Central Bank and the International Monetary Fund to the Eurogroup of 25 June, 2015, and comprised of two parts which make up their joint proposal, be accepted? The first document is titled "reforms for the completion of the current programme and beyond" and the second "Preliminary debt sustainability analysis".
Voters must check one of two boxes - "not approved/no" or, below it, "approved/yes"
Electoral workers have raced to get polling stations ready in time, with army helicopters being used instead of boats to rush ballot papers to the islands. Nearly 10 million people are eligible to vote.
Several European officials have complained in strong terms about Greece's abrupt decision to hold a referendum on the terms of a bailout offer they say is no longer on the table.
Greece's Syriza-led government was elected in January on an anti-austerity platform.
The European Commission - one of the "troika" of creditors along with the IMF and the European Central Bank - wants Athens to raise taxes and slash welfare spending to meet its debt obligations.
Lenders' proposals: Key sticking points
- VAT (sales tax): Alexis Tsipras accepts a new three-tier system, but wants to keep 30% discount on the Greek islands' VAT rates. Lenders want the islands' discounts scrapped
- Pensions: Ekas top-up grant for some 200,000 poorer pensioners will be phased out by 2020 - as demanded by lenders. But Mr Tsipras says no to immediate Ekas cut for the wealthiest 20% of Ekas recipients
- Defence: Mr Tsipras says reduce ceiling for military spending by €200m in 2016 and €400m in 2017. Lenders call for €400m reduction - no mention of €200m
Are you in Greece? How has the turmoil affected you? You can share your experiences by emailing email@example.com.
Please include a telephone number if you are available to talk to a BBC journalist.
Or WhatsApp us on +44 7525 900971.