Across the country of 11 million people -- on far-flung Aegean islands, in the shadow of the 2,400-year-old Parthenon in Athens, to the northern border shared with fellow EU state Bulgaria -- voters began casting their ballots.

The rest of Europe, and international investors, will be watching intently, unsure of the outcome that could greet them on Monday. Polls suggest both the 'Yes' and 'No' camps are neck-and-neck. Dozens of people were queuing outside schools and university buildings transformed into polling stations, with doors opening sharply at 7:00am local time (0400 GMT) and set to close 12 hours later.

"I'm voting 'No' because I think it's better for the country," said 80-year-old Michelis, first in through the doors of a school being used for the vote on Skoufa street in central Athens. "If we vote 'No' they'll take us more seriously," he said, adding that he was "not voting for myself, but for my grandchildren" and their future.

Theodora, 61, a retired journalist, said she was voting 'Yes' because "it's a 'Yes' to the European Union". Greece's youthful Prime Minister Alexis Tsipras, a radical leftist who came to power six months ago, has staked his political career on the plebiscite.

He announced it a week ago in a bid to break a five-month impasse with international creditors, insisting a 'No' vote would force a restructuring of Greece's massive debt and a softening of drastic austerity conditions.

- Supermarkets emptied -

But many who first backed him have swung to the 'Yes' camp, heeding warnings from EU leaders, notably European Commission chief Jean-Claude Juncker, that a 'No' result could see Greece expelled from the 19-nation eurozone -- a so-called "Grexit".

Greece was officially declared in default on Friday by the European Financial Stability Facility, which holds 144.6 billion euros (USD 160 billion) of Greek loans, days after becoming the first developed country to miss a debt payment to the IMF.

Tsipras's flamboyant finance minister, Yanis Varoufakis, on Saturday accused Athens's creditors of "terrorism" for trying to sow fear around the vote. He pointed out that no legal mechanism exists to force Greece out of what is meant to be an "irreversible" monetary union.

Greeks were nevertheless alarmed this week when the government imposed capital controls, closing banks and limiting daily ATM withdrawals to just 60 euros (USD 67).

The banks' liquidity was expected to dry up entirely in just one or two days' time unless the European Central Bank (ECB) injected funds quickly. Supermarket shelves have been emptied in the days leading up to the referendum.

"Most people are buying food now because they fear the worst," said Andreas Koutras, a 51-year-old Greek woman who works in finance in the capital.

Mothers, elderly men and university students were seen pushing heavily overloaded trolleys or coming out of shops weighed down by bags of food, with essentials such as sugar, flour and pasta top of the list.

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