Greek voters have decisively rejected the terms of an international bailout. The final result in the referendum, published by the interior ministry, was 61.3% "No", against 38.7% who voted "Yes".
Greece"s governing Syriza party had campaigned for a "No", saying the bailout terms were humiliating.
Their opponents warned that this could see Greece ejected from the eurozone, and a summit of eurozone heads of state has now been called for Tuesday.
Greek Prime Minister Alexis Tsipras said late on Sunday that Greeks had voted for a "Europe of solidarity and democracy".
"As of tomorrow, Greece will go back to the negotiating table and our primary priority is to reinstate the financial stability of the country," he said in a televised address.
"This time, the debt will be on the negotiating table," he added, saying that an International Monetary Fund assessment published this week "confirms Greek views that restructuring the debt is necessary".
But some European officials had said that a "No" would be seen as an outright rejection of talks with creditors.
Jeroen Dijsselbloem, who heads the eurozone"s group of finance ministers, said the referendum result was "very regrettable for the future of Greece".
Germany"s Deputy Chancellor, Sigmar Gabriel, said renewed negotiations with Greece were "difficult to imagine".
Mr Tsipras and his government were taking the country down a path of "bitter abandonment and hopelessness", he told the Tagesspiegel daily.