Leader of New Democracy and former Prime Minister Antonis Samaras has resigned.
François Hollande and Angela Merkel have called for a meeting of euro zone members to take place on Tuesday.
Greek Finance Minister Yanis Varoufakis says “Today’s ‘No’ is a no to a euro zone which acts as a cage for its people. It is a ‘Yes’ to a euro zone being a land of prosperity to its people.”
Senior German conservative Michael Fuchs says Tsipras has caused a “disaster”.
With more than 80 percent of the ballots counted, ‘No’ is ahead with some 61.54 percent of the votes, compared to 38.46 percent for the ‘Yes’ vote.
While the full results are yet to be announced, supporters of the ‘No’ campaign have already been celebrating on the streets of central Athens.
Waving Greek flags, jubilant demonstrators flocked to Syntagma Square, chanting anti-austerity slogans.
Earlier, Greek private TV stations estimated victory for the ‘No’ camp in Sunday’s referendum.