Banks in Greece and the country’s stock exchange will not open on Monday after people rushed to withdraw their cash.
Prime Minister Alexis Tsipras announced the closures after panic gripped the country in the lead-up to a referendum on bailout terms.
And it has emerged that members of the Greek Financial Stability Council have recommended that the banks stay shut for a total of six days.
The impact of that recommendation is unclear, but all banks will be closed for at least tomorrow, Mr Tsipras said.
Speaking in a televised address, the Prime Minister urged calm and maintained that bank deposits were safe.
He blamed European partners and the European Central Bank for forcing Greece’s hand.
And he said today’s developments would not halt the referendum plan next Sunday.
European creditors have refused a request to extend Greece’s international bailout beyond Tuesday, until after the referendum.
It means Athens is at risk of defaulting on its €1.5bn IMF payment, with membership of the eurozone hanging in the balance.
Mr Tsipras said: “The recent decisions of the Eurogroup and ECB have only one objective: to attempt to stifle the will of the Greek people.
“They will not succeed. The very opposite will occur: the Greek people will stand firm with even greater wilfulness.
“In the coming days, what’s needed is patience and composure. The bank deposits of the Greek people are fully secure.
“The same applies to the payment of wages and pensions – they are also guaranteed.
“In these critical hours, we must remember that the only thing to fear is fear itself.”
Sky’s Europe correspondent Robert Nisbet said the closures were an attempt to limit damage to banks as people rushed to withdraw money.
“This is a very serious development,” Nisbet said. “Things are looking particularly gloomy for Greece’s continued membership of the eurozone.”
As news of the closures spread, protesters gathered outside parliament in Athens.
President Barack Obama and German Chancellor Angela Merkel spoke on Sunday about the latest phase of the crisis.
Tomorrow, France and Germany will hold crisis meetings over the Greek crisis.
Earlier, the European Central Bank said it would maintain its emergency cash lifeline to Greece’s banks – but there would be no increase.