Greece's prime minister has set a date of July 5 for a referendum on the country's bailout deal with international creditors.
Alexis Tsipras made the announcement in a televised address to the Greek people early on Saturday, following an emergency cabinet meeting.
"The Greek government has been asked to accept a proposal that places new unbearable burdens on the Greek people," he said.
"Right now, we bear an historic responsibility concerning ... the future of our country. And this responsibility obliges us to answer [the bailout creditors'] ultimatum based on the sovereign will of the Greek people."
Tsipras has accused Greece's creditors of demanding new pension cuts, sales-tax increases and labour market reforms.
Al Jazeera's Barnaby Phillips, reporting from Athens, said Tsipras' announcement was "an extraordinary moment of drama in the Greek debt crisis", leaving many questions unanswered.
"Perhaps Tsipras is hoping that with a no-vote he could return to creditors and say 'I have the Greek people behind me, you have to offer better terms,'" he said.
Ask for extension
Tsipras said he would ask creditors on Saturday for an extension "of a few days" to Greece's bailout programme, which expires on Tuesday.
Without an agreement on its international bailout, Greece faces the threat of running out of cash, defaulting on its loans and, possibly, leaving the euro currency.
Without a $8.1bn bailout loan instalment - which would only be available if there is a deal - the country will be unable to make a $1.8bn payment to the International Monetary Fund on Tuesday, and even bigger payments later in July.
By essentially defaulting on its debt mountain, Greece would likely see its banks collapse, as they depend on emergency European Central Bank funding.
The government could soon run out of cash, face huge difficulties in paying pensions and civil servant salaries - and that could force it to leave the eurozone and adopt a weak national currency.
Greece's development minister urged the nation to vote against the bailout deal.
Panayiotis Lafazanis said Greeks will answer "with a resounding no" in the vote.
Worried Greeks have been pulling their money out of banks for months, and an estimated $4.5bn left Greek banks last week.
Long queues were seen forming outside several Athens cash machines and petrol stations late on Friday and early Saturday.