Greeks are voting on Sunday whether to accept worsening austerity or back the government’s rejection, in a referendum that presents the biggest challenge to the running of the euro.

Greeks are voting on Sunday whether to accept worsening austerity or back the government's rejection, in a referendum that presents the biggest challenge to the running of the euro since its adoption and risks sending shock waves through the world’s financial markets.

Across the indebted country of 11 million people voters were set to cast 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.

The nationwide ballot was taking place at the end of a week of unending drama that saw Greece close its banks, ration cash, fail to repay the IMF and lose billions of euros when its bailout program expired.

The vote is on the last terms offered to Greece before its prime minister, Alexis Tsipras, abandoned talks with his country’s lenders last weekend, saying their conditions would only exacerbate the plight of a country whose economy has already shrunk by a quarter.

The prime minister’s decision to call the vote prompted outrage among opposition politicians who favor a yes vote.

They have argued that the offer on which the referendum is based was withdrawn when the bailout program ran out and that Tsipras’s Syriza party has rigged the ballot by putting both options on one ballot sheet with the no option first.

But a claim that the vote was unconstitutional was thrown out by Greece’s highest court on Friday.