UPDATED 8.39am: Greek voters have overwhelmingly rejected international creditors' tough bailout terms in a landmark referendum, reigniting fears of a "Grexit" from the eurozone.

With near-complete results showing the 'No' vote winning by more than 61 per cent, Prime Minister Alexis Tsipras says his negotiating position to secure a new debt deal has been greatly bolstered.

LISTEN: Above, Helena Smith is the Guardian's Greece correspondent and she talks to Rachel Smalley on KPMG Early Edition about the Greek referendum and what the polls are saying.

MORE: Andrew Kelleher - Referendum in Greece

In a televised address after the referendum, Tsipras insisted the vote did not mean a break with Europe. 

He emphasised that euro membership is meant to be "irreversible", with no legal avenue to boot a country out.

"Together we have written a bright page in modern European history," he said.
"This is not an mandate of rupture with Europe, but a mandate that bolsters our negotiating strength to achieve a viable deal."

Tsipras claimed the creditors - the ECB, the European Commission and the International Monetary Fund (IMF) - would finally have to talk about restructuring the massive, 240 billion euros ($A352.66 billion) debt, Greece owes them.

"This time, the debt will be on the negotiating table," he said.

Eurozone leaders and officials - some of whom viewed the plebiscite as an in-out vote on Greece's membership in the euro - scrambled to work out their response to Sunday's `No' vote.

German Chancellor Angela Merkel and French President Francois Hollande hastily called a European summit for Tuesday after stating the Greeks' decision must "be respected".

Tsipras has "torn down the bridges" between Greece and Europe, Merkel's deputy chancellor, German Economy Minister Sigmar Gabriel, said.

Despite the Tsipras' optimism, new bailout negotiations now were "difficult to imagine", he said.

European Commission president Jean-Claude Juncker - who had said a Greek 'No' would be "no to Europe" - was to speak to the European Central Bank (ECB) and eurozone finance ministers on Sunday and Monday.

The head of the Eurogroup of eurozone finance ministers, Jeroen Dijsselbloem, called the Greek 'No' result "very regrettable for the future of Greece".

Thousands of pro-government supporters cheered and hugged each other in central Athens in celebration, although some other Greeks expressed pessimism that Tsipras would be able to deliver on his promises.

Greece is teetering on the brink of financial collapse. If it doesn't receive cash and loans soon from European institutions, it could still be forced to resort to government IOUs or a return to drachmas to keep its economy running.

Last Tuesday, the country defaulted on a 1.5-billion-euro repayment to the IMF, becoming the first developed country to fall into arrears to the institution.

Greek banks are now reportedly almost illiquid after a bank run by panicked customers in the lead-up to the referendum.

A weeklong closure of the banks and capital controls that included restricting daily ATM withdrawals to just 60 euros and blocking money transfers abroad slowed the outflow.

Reacting to the referendum outcome, analysts at Barclays Research said a Grexit was now "the most likely scenario", while Deutsche Bank said the decisive 'No' meant the "probability of unpredictable outcomes to the Greek crisis has again materially increased".