Greeks voted by a large margin to reject the austerity terms of an aid package from international creditors, an official projection of the final result of Sunday's referendum showed.

"The assessment of Singular Logic is that the result in favour of 'No' will exceed 61 percent," an official from Singular Logic, which processes the results for the interior ministry said.

Only 39 percent were estimated to have voted in favour.

The result would hand a big victory to Prime Minister Alexis Tsipras, who urged voters to say 'No' to an aid package he called a national "humiliation" and an "ultimatum" from creditors.

But pro-euro parties and euro zone policymakers have warned a 'No' would be tantamount to rejecting talks with creditors, setting Greece on a path out of the euro.