Greece's government and international creditors have raised the stakes over a weekend referendum seen as decisive for the nearly insolvent EU country's political and financial future.

The radical left government of Prime Minister Alexis Tsipras "may very well" resign if Greeks spurn its call to vote 'No' in Sunday's plebiscite, Finance Minister Yanis Varoufakis said on Thursday.

But exasperated EU leaders warned the eurozone would be plunged into "unknown" territory unless a 'Yes' vote prevailed.

The International Monetary Fund highlighted the deteriorating situation in Greece by slashing the country's growth forecast this year from 2.5 per cent to zero.

It also estimated Greece needs at least 50 billion more euros to stabilise its finances even under existing creditor plans.

Accelerating austerity, closed banks this week and the prospect of crashing out of the eurozone - and maybe even the European Union - are taking their toll on Greece's population of 11 million.

"Now it's only the banks. But if there's a run on supermarkets, and fuel starts running out, it could lead to riots, to chaos, even to a coup by the sort of military junta which seized the country in 1967," said Georgiadis Aris, an Athens lawyer.

The upcoming referendum has split the country, and is fracturing the ruling coalition made up of Tsipras's Syriza party and the Independent Greeks party. One lawmaker from the latter was expelled from the parliamentary bloc on Thursday for urging a 'Yes' vote.

In obfuscatory language, the plebiscite asks whether Greeks are willing to swallow yet more austerity in return for a bailout that expired just days ago.

But EU leaders have framed the vote as a consultation on whether Greeks want to stay in the euro or not.

If Tsipras's government obtains its 'No' vote, it would send the eurozone "into the unknown," French President Francois Hollande said on an African visit.

While there is no legal mechanism to directly force Greece out of the eurozone, it could end up as a zombie member with no euros, analysts say.

The head of the Eurogroup of finance ministers, Jeroen Dijsselbloem, said: "In case of a 'No', Greece's situation will become exceptionally difficult."

Duelling pro-'Yes' and -'No' demonstrations have been held in Athens this week, gathering thousands, and more are to take place on Friday.

Greece's top administrative court, the Council of State, is also to rule Friday on the legality of the rushed referendum, announced only last weekend.

Voter surveys have given no reliable indication of how the plebiscite, if held, would fall.One poll leaked to Greek media on Thursday put the 'No' camp at 43.2 per cent, and the 'Yes' leading with 47.1 per cent. But the survey company involved, GPO, issued a statement warning the poll was only "fragmentary" and not meant to be made public.

The world's financial markets and Greece's creditors - the European Commission, European Central Bank (ECB) and the IMF - have largely stepped back to watch the outcome.

"Greece may be out of the spotlight for the time being, but it should not be forgotten, and this weekend could be the end of the line for Greece and the euro," said David Madden, market analyst at IG trading group.

Greece's finance minister Yanis Varoufakis told Australian radio his country was on a "war footing" to get the referendum ready in time.

Varoufakis also said "yes, we may very well" resign and handover to a caretaker administration.