The draft law on Greece's agreement with its lenders, including tax rebates, benefits and perks for public servants, faces a late-night ballot.
There were signals that ruling party Syriza was heading for a split but the vote is still expected to pass.
Prime Minister Alexis Tsipras has said he is confident of success and that the deal will bring economic security.
He needs MPs to vote for a Memorandum of Understanding (MoU) to be adopted before eurozone ministers can endorse the draft deal worth about €85bn (£61bn; $95bn).
They meet on Friday.
The next crunch deadline for Greece is then 20 August, when it must repay about €3.2bn to the European Central Bank (ECB).
In two prior votes on bailout reforms, Mr Tsipras faced rebellions from his own party with more than 30 of Syriza's 149 MPs refusing to approve the latest tax increases, pension cuts and market reforms.
On Thursday, the bill was argued over for about nine hours.
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The leader of Syriza's far-left faction, former energy minister Panagiotis Lafazanis called for a new movement to fight the deal.
The government said he had clearly decided "to choose a different path from that of the government and Syriza".
It has defended the controversial new programme as tough but essential if the country is to avoid financial collapse.
Analysis: BBC diplomatic correspondent Paul Adams in Athens
Once again, the Greek parliament is the scene of long, passionate discussions, destined to go on all night. Earlier, during committee discussions, emotions were already running high. Now, as MPs wrangle over procedure, the main debate hasn't even started. It'll be many hours before the bailout bill comes to a vote.
With the main opposition parties generally supporting the bill, the outcome is not seriously in doubt. Parliament will approve an astonishing array of reforms which are likely to affect almost every aspect of economic life in Greece. Farmers will lose subsidies and see their rates of tax double. The self-employed will also be hit hard. There'll be pension reform, too, and a review of Greece's entire welfare system. The scope is breathtaking.
But getting all this through parliament will come at a cost. Mr Tsipras, who once vowed to resist any more austerity, faces a rebellion among hardline members of his own Syriza party. They're threatening to break away and there's already talk of fresh elections in the autumn.
The rebels, including parliamentary speaker Zoe Konstantopoulou and former finance minister Yanis Varoufakis, insist the government should make good on its electoral promise to reverse spending cuts and tax rises.
However, lawmakers are expected to approve the MoU by a comfortable margin with the help of opposition parties.
Meanwhile, on Thursday, data showed the Greek economy grew by 0.8% in the second quarter of the year, confounding expectations of a steep contraction.
Until the figures were released, the economy had been forecast to shrink again this year by between 2.1% and 2.3%.
