Greek prime minister Alexis Tsipras has called for a 30% reduction of the country's debt and a grace period of 20 years as polls show the country is split ahead of the 5 July bailout referendum.
In a televised address to the nation, Tsipras urged Greeks to vote 'no' to a cash-for-reforms deal put forward by troika creditors and cited an International Monetary Fund report that supported his position.
He said: "Yesterday an event of major political importance happened. The IMF published a report on Greece's economy, which is a great vindication for the Greek government as it conforms the obvious: Greek debt is not sustainable."
Tsipras also urged Greeks to be responsible and not panic as the situation in Greece becomes polarised between the 'yes' and 'no' camps.
"Let us calmly go to the polls and make our choice by weighing the arguments, not the slogans. No to those trying to incite panic, prevent you from making a choice calmly and responsibly about your future."
He also reiterated accusations of "blackmail" levelled at the European Union, the IMF and European Central Bank.
The speech came amid new polling that showed the outcome of the referendum is too close to call.
The latest opinion poll from Greek newspaper AVGI suggests 43% of Greeks think the government should reject the bailout terms put forward by troika creditors, with 42.5% in the 'yes' camp. 9% were undecided.
Greek opinion on the cash-for-reform conditions appears to have changed between the announcement of the referendum and the latest poll as the plebiscite is now seen as being about the country's continued eurozone membership.
James Nixon from Oxford Economics said: "Even if Sunday's referendum delivers a 'yes' vote, the challenges to finding a solution that will preserve Greece's euro membership are substantial. The IMF has suggested that any new bailout arrangement for Greece will need to be around €60bn (£43bn, $67bn) to cover Greece's financing needs over the next three years and will need to include a significant debt write down. It is not obvious that a programme of this size would be approved by European parliaments."
Tsipras' Syriza government is campaigning for a 'no' vote on the basis that it can get a better deal from Brussels. Both he and Finance Minister Yanis Varoufakis have pledged to resign if the Greeks vote 'yes'.
Failure to meet a 30 June IMF debt payment deadline led to the government closing banks until after the referendum and imposing capital controls, permitting the Greeks to withdraw a limit of €60 (£43, $67) a day.