Carrying banners calling for a "no" vote in the forthcoming referendum on bailout conditions set by the country's creditors, protesters gather in front of the Greek parliament in Athens on June 29, 2015. Getty Images/AFP/Louisa Gouliamaki

Greek Prime Minister Alexis Tsipras threatened to resign over the outcome of a referendum scheduled for Sunday that would determine Greece’s financial future.

Tsipras said in a national address that if the country's citizens voted in the referendum to accept the terms of the bailout being offered by Greece’s international creditors, which would also call for Greece to institute austerity reforms, then his ruling Syriza party would not stay in power. Greece and its creditors have entered a new round of last-minute talks, hours before it is set to default on a $1.72 billion loan repayment to the International Monetary Fund (IMF), the BBC reported, citing local media.

“The bigger the turnout and the wider the no-vote to that deal, the bigger the possibility for a substantial restart of negotiations, so that we can take a course of viability and reason,” he said, according to Euronews. On Sunday, Tsipras accused the creditors of “blackmail.”

He added that a strong vote to reject the bailout's terms would strengthen Greece's position in its demand for concessions from its creditors, allowing him to try to avert painful pension reforms that have been called for.

"I don’t believe that they want to kick us out of the euro and they won’t. They won’t, because the cost would be huge,” he said, referring to the possibility of Greece leaving the eurozone, known as “Grexit.”

However, he added, if Greece were to agree to the terms of the bailout, then he and his party would not remain in power. "If the Greek people want to proceed with austerity plans in perpetuity, which will leave us unable to lift our head... we will respect it, but we will not be the ones to carry it out," he said.

Tens of thousands of people reportedly gathered outside the parliament in Athens to show support for Syriza’s proposal. A rival protest by those who support the bailout terms is due to occur later Tuesday.

Greek Finance Minister Varoufakis has confirmed that the country will not pay IMF back on Tuesday, when its debt payment comes due, AP reported.

Greece has shut its banks for one week and imposed capital controls and withdrawal limits as its citizens rushed to stock up on cash and rations, fearing the events of the days to come.

Meanwhile, Greece’s opposition called on Tsipras to call off the referendum and “stop the destruction.”

"We will not become beggars in the Balkan neighbourhood. The 'yes' to Europe will win," opposition leader Stavros Theodorakis, who heads the To Potami party, said on Monday, according to the Greek Reporter. "We call on Tsipras to find the courage to tell the Greek people the truth. To set aside opportunism. To withdraw the referendum and enter into an agreement with the European Union," he said.

Meanwhile, European Commission President Jean-Claude Juncker called on Greek voters to oppose the measure, insisting that Greece exiting the eurozone did not have to be the outcome and that the new terms proposed by creditors would be more equitable, BBC reported.