Polling booths closed at 16:00 GMT on Sunday, with opinion polls showing the nation evenly split between "Yes" and "No" vote.
With 10 million Greeks eligible to vote, initial polls broadcast after polling stations closed suggested a narrow win for the "No" vote.
German Chancellor Angela Merkel and French President Francois Hollande announced after the voting had ended that they would meet in Paris on Monday evening to discuss the result.
Polling booths opened at 7am local time (04:00 GMT) on Sunday, with opinion polls showing the nation of 11 million people evenly split between "Yes" and "No" vote. The first projected result is expected at 9pm local time.
Prime Minister Alexis Tsipras has urged people to vote "No", saying it would strengthen his left-wing government's hand in talks with international creditors who are owed billions of euros.
Tsipras remained steadfast after casting his vote at a polling station in Athens on Sunday morning.
"No one can ignore the message of determination of a people taking its destiny in its own hands," he told reporters.
On Friday, more than 25,000 people welcomed Tsipras at a rally in Athens where he sought to revive support for the "No" vote.
A rival rally of 22,000 "Yes" supporters shouted pro-European slogans and voiced fears of a so-called "Grexit" from the eurozone and a return to Greece's former currency, the drachma, if Tsipras got his way.
