Reports indicate that the prime minister could call a confidence vote, which may trigger fresh elections and jeopardize the bailout.
The bill lawmakers will vote on late on Wednesday covers rules for dealing with failed banks and speeding up the justice system.
Anti-establishment protesters threw dozens of petrol bombs at police in front of parliament.
Prime Minister Alexis Tsipras spent the day with his cabinet drafting a last-ditch package of tax rises, pension reforms and economic liberalization measures.
Getting the Greeks to say no was the easy part. Now Tsipras must quickly mend fences with his European partners.
In the EU, there is supposed to be no turning back, but what happens when a country can't keep up?
Latvia, Ireland, Spain, Portugal and Slovakia endured austerity and are in no mood to allow Greece off the hook.
Tsipras said negotiations would continue for a better deal with international creditors after the vote.
The poll put the 'Yes' camp on 44.8 percent against 43.4 percent for the 'No' vote.
Greek PM Tsipras says a 'no' vote would step up pressure on creditors to give the country an economically viable agreement.
Greece owes official lenders 242.8 billion euros ($271 billion).
The missed payment would bring Greece closer to a Euro exit if it prompted the European Central Bank to cut off emergency funding.
After an initial wave of selling, European stocks and the euro recovered some ground.
Greece has less than 48 hours to pay back 1.6 billion euros ($1.77 billion) of International Monetary Fund loans.
The eurozone common monetary policy is not fit for purpose for all eurozone member states