Athens: Greek banks – and their customers – should brace for a "difficult day" on Tuesday if the Athens government does not reach a deal with its creditors, senior bankers have warned.

However the government, led by prime minister Alexis Tsipras into critical talks in Brussels, is also aware that if a deal is made but does not satisfy its supporters at home, it may lose hold of power in the fractured, anxious parliament and face renewed street protests.

One such anti-austerity protest, thousands strong, gathered in Athens' central Syntagma square on Sunday night.

Many knew it could be Greece's last evening before it is plunged into economic chaos – but it was a balmy summer night, and the cafes and street stalls were doing a booming business. Past 10pm the protest showed no sign of flagging.

Under her now-moderately-famous "Unf*ck Greece" placard (U2 use it in their current tour), veteran protester Panagiota Bleta was straightforward about exactly how her country should be fixed.

"We need to fight austerity," she says. "The whole of Europe, not just Greece. Nations can't live under these conditions."

Austerity has left her country ravaged by unemployment and many remaining wages below the poverty line, she says. Money that comes into Greece leaves straight away, back into the pockets of its creditors.

"These are war conditions."

She was unfazed by the idea of a "rupture" with the Eurozone, or even the EU.

"The cost of living is rising, business life has been 'disappeared', there's no development," Ms Bleta said. "If we don't succeed [in getting a better deal] it's not worthwhile to stay in Europe. It's like we bought a bad franchise."

Around her on the city's main square, round the corner from the Parthenon​, thousands of young and old milled about, chatting about this or that, clapping along to the music, arguing passionately about politics, or taking selfies​ in front of banners.

It was not an angry protest, but a determined one.

They knew time is running out, this time maybe for real.

"I think there are more expectations now, on every side – everyone knows this is coming to an end, one way or another," said political blogger Michalis Panagiotakis​.

The Greek crisis has been a moment that never quite happens. Eurocrats​ are experts at kicking cans down roads, and each time Greece's economy has circled the plughole there has been a brief splash of cash from the European money tap.

But Greece's newly-elected Syriza​ party brought a new bravado to negotiations. Insults have been thrown on both sides.

Finance minister Yanis Varoufakis​ wrote in a fiery blog post last week that Europe's insistence on further cuts to pensions was "tantamount to turning families into the streets".

And the IMF's Christine Lagarde complained "the key emergency is to restore the dialogue with adults in the room" – responding to Mr Tsipras' claim the lender bore "criminal responsibility" for his country's problems.

On Sunday the phone lines between Athens, Berlin, Paris and Brussels ran hot, as Mr Tsipras​ and other European leaders tried to stitch together a bailout deal ahead of the meeting.

If this summit does not strike a deal, then there is little hope of avoiding a Greek default on a €1.6 billion ($2.3 billion) IMF loan, due to be repaid by the end of June. If the loan is not repaid, Greece is likely to crash out of the euro and perhaps even the EU.

International lenders are holding "hostage" €7.2 billion of new bailout cash, which will be released when Greece agrees to an economic reform package.

Leaders could reach a broad, "in principle" deal, and hammer out the details at the very last minute .

Neither side wants Greece to leave the eurozone. And there is said to be just a few billion euros difference in the reform packages being proffered.

But the head of the Bank of Greece warned bankers on Friday to brace for a "difficult day" on Tuesday if there is no deal with creditors, senior bankers told Reuters.

Bank of Greece Governor Yannis Stournaras​ called a special meeting with top bankers on Friday to discuss the mounting crisis in the banking sector after a flood of withdrawals that passed a billion euros a day towards the end of the week.

The bankers said Mr Stournaras​ did not talk about capital controls being imposed on Greek banks, however "if there is no deal, Tuesday will be a difficult day and decisions will have to be taken, is what we were told," one of the bankers said.

"If there is no deal on Monday, God knows what Tuesday will look like. Even the ECB is not sure on the road map thereafter," another told Reuters.

Another €1 billion were already booked to leave the country's banks on Monday – customers who want to withdraw large sums from their accounts usually have to file pre-orders to ensure that banks have enough cash.

Around €4.2 billion were taken from Greek banks last week.

However Syriza's​ negotiators are also painfully aware that concessions that would be broadly acceptable to the Greek public may prove unacceptable to its own MPs.

Depending on the degree of movement, they could lose as many as 20 parliamentarians from their fragile coalition.

"It depends very much on the deal that is done," Mr Panagiotakis said. "It's political, it's about who has control. If Syriza​ is seen as giving in to their demands, then they have no reason to continue in government – it's a grassroots party, until recently some of its members were fighting in the streets with tear gas.

"Syriza​ – and Greece – doesn't want a deal where something is given now but taken away again in three months time. Syriza​ wants a deal, even with compromises, that allows them to continue with policies without being kept hostage.

"But the other side, they are concerned that Syriza​ would go back on a deal too."

But if they secure a lasting deal, rather than a temporary fix, they will have achieved what many thought impossible.

"My neighbours, friends and family say if Syriza​ can deliver just 20 per cent of what it promised, it will be in power for 20 years," Mr Panagiotakis says.

With Reuters