Millions of Greeks are voting in a crucial referendum on whether to accept the terms of an international bailout.
Polling stations opened at 07:00 local time (04:00 GMT), with the first results expected on Sunday evening.
After a frenetic week of campaigning, the two opposing sides held huge rallies in Athens on Friday.
The government has urged voters to vote “No”, but opponents warn this could see Greece ejected from the eurozone.
Greek Finance Minister Yanis Varoufakis told local media on Saturday that the EU had “no legal grounds” to throw Greece out of the euro.
On the eve of the referendum, he accused Athens’ creditors of trying to sow fear around the vote. He told Spain’s El Mundo newspaper (in Spanish): “Why did they force us to close the banks? To instil fear in people. And spreading fear is called terrorism.”
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 is a member of the eurozone. There’s no doubt about that,” he told German newspaper Bild.
“Whether with the euro or temporarily without it: only the Greeks can answer this question. And it is clear that we will not leave the people in the lurch.”
Greece’s current bailout programme with the European Commission, International Monetary Fund (IMF) and European Central Bank (ECB) ran out on Tuesday.
Banks have been shut all week, with limits imposed on cash withdrawals.
Mr Varoufakis said that the banks in Greece would reopen on Tuesday whatever the outcome and that Greek Prime Minister Alexis Tsipras would still reach an agreement with creditors if the result was “No” in the referendum.
Ballot paper question
“Must 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 been racing 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. Opinion polls on Friday suggested that the country was evenly split.
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 left-wing Syriza government was elected in January on an anti-austerity platform.
The European Commission, the European Union’s executive arm – one of the “troika” of creditors along with the International Monetary Fund and the ECB – 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
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