Greek PM Lucas Papademos is set to meet the party leaders of his coalition to try and win support for a proposed 130bn euros ($171bn; £108bn) EU rescue.
Mr Papademos wants their backing for reforms demanded by the IMF and EU as a condition of the bailout.
Eurozone ministers had hoped to meet on Monday to finalise the bailout, Greece’s second, but that meeting has now been cancelled.
The money must be in place by mid-March if Athens is to avoid a debt default.
It is hoped Mr Papademos can reach a deal with party chiefs by Sunday night.
“The moment is very crucial,” said finance minister Evangelos Venizelos on Saturday.
“Everything should be concluded by tomorrow [Sunday] night.”
Athens faces loan repayments to private lenders of 14.4bn euros ($19 billion) on 20 March.
The BBC’s Mark Lowen, in Athens, says Greece cannot afford to lose the bailout package and a lot now rides on these talks.
But, our correspondent says, that with potential elections in April, the parties in Mr Papademos’ coalition are wary about being seen to be associated with the austerity measures being demanded by the EU.
EU officials have expressed frustration with Greece over delays in backing the terms of the latest rescue package, which is being put together by the European Union, the International Monetary Fund and the European Central Bank – the so-called “troika”.
“There is great impatience and great pressure not only from the three institutions that make up the troika but also from eurozone member states,” said Mr Venizelos, after what he described a “very difficult” conference call with his eurozone counterparts.
Reforms that international lenders want to see include a lower minimum wage, the removal of a “13th and 14th month” extra salary which is paid to workers as an annual bonus, and the liberalisation of workplace regulations.
Opponents say that more cuts will worsen living conditions which have already been affected by two years of austerity measures.
Unless Greece promises to implement reforms, the eurozone ministers say Greece will not be able to go ahead with a plan to restructure its privately-held debt.
Greece has prepared a debt plan with private creditors to halve the value of Greek debt and in return receive new, 30 year bonds with an average interest rate of less than 4%.
The restructuring is to help cut Greek debt to 120% of GDP in 2020 from 160% now.
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