Greek opposition parties are demanding the resignation of the government of PM Alexis Tsipras following his move to call a referendum on the latest bailout deal next Sunday.

Socialist party PASOK, which was the junior coalition partner in the previous government led by Antonis Samaras of New Democracy, slammed at Tsipras warning he was "unable to take responsible decisions" and urging him to step down.

Samaras himself is quoted by the Greek Reporter as saying the PM has proposed "a rift with all [of Greece's] partners and an exit from the euro."

A motion is due in the Athens Parliament to Saturday on whether a referendum should take place next week.

Tsipras announced in the small hours of Saturday, after returning from crunch talks in Brussels, that a national poll will be held on the proposals tabled by the European Union (EU), the European Central Bank (ECB), and the International Monetary Fund (IMF).

 “I call on the Greek people to rule on the blackmailing ultimatum asking us to accept a strict and humiliating austerity without end and without prospect,” Kathimerini quotes him as telling citizens in a televised address.

Despite being against them - and having rejected them at the Friday meeting - he would accept anything chosen by the Greek people, the Prime Minister made clear.

The conditions turned down by Athens would have helped disburse EUR 15.5 B in funding, with EUR 1.8 B of them available immediately.

Creditors also offered a 5-month extension to the bailout.

German Chancellor Angela Merkel, often said to be among the hardliners in austerity policy applied to Greece, said the offer was "extremely generous."

This comes as Greece is due to deliver on a loan installment worth EUR 1.5 B or face default and possibly leave the Eurozone.

Finance Minsiter Yanis Varoufakis earlier said Athens was determined to stay in the single-currency area and had done its best with regard to lenders' "strange demands".

But he also described the current terms offered by creditors as unviable, whereas Tsipras himself even used the word "humiliation".

How (or if) reforms will be carried out in the pension system and public-sector wages is the bone of contentions, alongside taxation and benefits policy. Greece is also calling for a debt relief, a demand which has not yet met a positive response.

New meetings are due later on Sunday between Greek and EU officials, including one of Greece's Deputy PM Yannis Dragasakis and Deputy FM Euclid Tsakalotos.

An emergency Eurogroup session - the fifth one in just six days - has also been called in.