Michalis Kariotoglou, chairman of the IT company overseeing the referendum results, said: “We have currently gathered the results of 3,500 polling stations.
“Based on this, the Singular Logic estimates that the result will be No and it is going to take more than 61 per cent.”
Crowds of No campaigners gathered in Syntagma Square in Athens, blowing horns, dancing and waving the Greek flag, as the votes came in.
Greek interior minister Nikos Voutsis said: “We are sending a message of determination, in the birthplace of democracy, a message of co-decisions, regardless of every person’s choice, which means that our country, our people, want to move forward and put their seal on current developments.”
Government spokesman Gabriel Sakellaridis said negotiations with the lenders over debt restructuring that would allow Greece to stay inside the eurozone would resume immediately.
He said: “The negotiations which will start must be concluded very soon, even within 48 hours. We will undertake every effort to seal it soon.”
Greek banks, which have been closed all week and limiting cash withdrawals, are expected to run out of money within days unless the European Central Bank provides emergency funds.
Finance Minister Yanis Varoufakis is due to meet top Greek bankers later on Sunday to try to restore liquidity to the banking system.
The European Central Bank, which holds a conference call on Monday morning, may be reluctant to increase emergency lending to Greece after voters rejected a package of austerity and economic reform which creditors say is essential to make the country’s public finances viable.