Tens of thousands of Greeks rallied on Monday to back their leftwing government's rejection of a tough international bailout after a clash with foreign lenders pushed Greece close to financial chaos and forced a shutdown of its banking system.

With a popular referendum on the bailout planned for Sunday, Prime Minister Alexis Tsipras put his own position on the line, saying he would respect the result of the vote but would not lead a government to administer "austerity in perpetuity."

"If the Greek people want to have a humiliated prime minister, there are a lot of them out there. It won't be me," he said in an interview on Greek state television as one of the biggest rallies seen in Athens in years was taking place.

The show of defiance came at the end of a day that started with stunned Greeks waking up to face shuttered banks, long supermarket lines and overwhelming uncertainty over Greece's future in the euro zone.

European leaders and policy makers, wrong-footed by Tsipras' shock announcement of the referendum in the early hours of Saturday morning, warned that it would be a plebiscite on Greece's future as a member of the single currency.

With Greece hours away from defaulting on a 1.6 billion euro (£1.14 billion) loan from the International Monetary Fund, the crisis has escalated quickly.