Thousands of Greeks staged rival rallies on Friday ahead of a weekend referendum that may decide the country’s future in Europe’s single currency, with polls showing voters almost evenly split.

At least 20,000 people turned out at each rally, at the end of a week of high drama following the government’s rejection of an aid deal with international creditors and the closure of banks.

With Greeks asked to decide whether to accept or reject the tough terms of the bailout, three opinion polls had the “Yes” vote narrowly ahead; a fourth put the “No” camp 0.5 per cent in front, but all were well within the margin of error.

With tension building, police firing stun grenades briefly scuffled with a few dozen black-clad people carrying red flags, often carried by anti-establishment radicals, on the capital’s central Syntagma square. The violence appeared to be isolated.

Prime minister Alexis Tsipras exhorted Greeks to reject the deal, dismissing warnings from Greece's European partners that to do so may see the country leave the euro zone, with unforeseeable consequences for Greece, Europe and the global economy.

Vindicated

In a televised address, Mr Tsipras said a report by the International Monetary Fund which argued that Greece's massive public debt could not be sustained without significant writedowns vindicated his advice to reject the lenders' terms.

Repeating his assault on European partners he accused of blackmailing and issuing ultimatums to Greece, Mr Tsipras called for calm ahead of Sunday’s vote.

“On Sunday what is at stake is not Greece’s membership of Europe, what is at stake is whether blackmail will lead us to accept the continuation of a policy which the lenders themselves recognise is a dead end,” he said.

“On Sunday what is at stake is whether we will give our consent to the slow death of the economy.”

Finance minister Yanis Varoufakis called the IMF report "music to our ears".

A GPO poll broadcast by Mega TV put the “Yes” vote on 44.1 percent to 43.7 percent against.

Another, by the respected ALCO on behalf of the Proto Thema newspaper, had those in favour 0.6 points in front.

A third published in Ethnos newspaper said 44.8 per cent would endorse the deal and 43.4 percent would vote “No”.

A fourth, for the Agvi newspaper, was the only one that put the “No” vote fractionally ahead, 43 per cent to 42.5 per cent.

European politicians fired fresh warnings of the costs of a "No" vote in a plebiscite called at just eight days' notice after the breakdown of talks with the European Commission, the IMF and the European Central Bank.

Commission president Jean-Claude Juncker and German finance minister Wolfgang Schäuble dismissed Mr Tsipras’s version that his government would be able to move smoothly to negotiate more favourable terms if Greeks backed his rejection.

“If the Greeks will vote No, the Greek position is dramatically weakened,” Mr Juncker told a news conference.

Mr Tsipras is betting Europe will compromise rather than let Greece slip out of the euro zone, even though the continent’s leaders say a “No” vote would signal its exit.

But behind the rhetoric, there were more concrete signs of the pressure Europe can exert on Greece.

The euro zone’s rescue fund, Greece’s largest creditor, said it was reserving the right to call in €130.9 billion of debt ahead of time after Athens defaulted on an IMF loan.

Completely open

Given a volatile public mood and a string of recent election results that ran counter to opinion poll predictions, the result of the vote is in effect completely open.

With banks shuttered all week, cash withdrawals rationed and commerce seizing up, it could decide whether Greece gets another last-ditch financial rescue in exchange for more harsh austerity measures or plunges deeper into economic crisis.

There has been little time for campaigning but Mr Tsipras is due to address a mass rally of “No” supporters in Athens’ central Syntagma Square outside parliament on Friday evening, while “Yes” campaigners rally at the old Olympic Stadium.

His opponents have pointed to the fact that the referendum is on a deal that is no longer on the table, accusing him of recklessly endangering the country’s future.

Mr Tsipras and Mr Varoufakis have said they would quit if voters ignore their call to vote "No". That would lead to a scramble to either try to put together a national unity government to negotiate a loan deal or call new elections by September.

But even if European leaders are willing to sit down immediately with an Athens government, it may be several weeks before a new bailout is sorted out, European Commission vice-president Valdis Dombrovskis told Germany's Die Welt daily.

Any decision on whether to reopen the crippled banks will depend on whether the European Central Bank is ready to restore the emergency funding they need to stay afloat.

That is far from certain as ECB Governing Council member Vitor Costancio made clear when asked whether funding would be restored. "I cannot in advance answer that question," he said, adding that a 'No' vote would make agreement difficult.

Greece’s European partners have made clear they regard the vote as a choice of whether Greeks want to stay in the euro, even though its exit would change the nature of a 15-year-old currency union intended to be unbreakable.

Reuters