Lowering its annual pension bill, one of the most expensive in the euro zone, is a condition for Athens to qualify for a fresh installment of the bailout worth up to 86 billion euros ($98.3 billion) it signed up to almost a year ago.
Greeks, incensed at years of austerity, declared a 48-hour strike starting on Friday. Major labour unions were planning protests outside Greece's parliament on Sunday, when the reforms are due to be put to the vote.
Tsipras, who is clinging to a slim majority of 3 seats in the 300 member parliament, appealed to members of his Syriza party to approve the package. Whether Greece signed up to a bailout or not, he said, the reforms were still needed.
"The necessity to make an intervention into the pension system must be understood by all. It was a necessity borne from the need to make it viable Without this intervention, the pensions system would collapse."
In the meantime, the head of the International Monetary Fund, Christine Lagarde, urged euro zone finance ministers to start talks on Greece's debt relief together with discussions on reforms, according to a letter published by the Financial Times.
Tsipras has called for debt relief of some form to help Greece as it introduces the reforms.
'NAIL IN THE COFFIN'
Friday's strike left ships docked at port, disrupted public transport and kept civil servants and journalists off the job.
Greece's largest labour union, the private sector GSEE, said the reforms, now pending approval in parliament, were the "last nail on the coffin" for workers and pensioners who have sacrificed enough after six years of austerity.
"They are trying to prove to the Eurogroup that they are good students but they are destroying Greece's social security system," a GSEE official said, referring to euro zone finance ministers who are due to meet on Monday.
Civil service union ADEDY held rallies in central Athens on Friday. Thousands of protesters with the Communist-affiliated group PAME marched before parliament holding banners that read: "Rise up now!" and "Resist".
Athens hopes the measures, due to be voted on in parliament on Sunday, will help persuade creditors to approve the release of bailout cash.
A tranche of more than 5 billion euros is overdue, after talks faltered over the pace of reforms. The Eurogroup is expected to discuss the stalemate on May 9 and long-desired by Athens debt relief measures.
Greece needs the bailout funds to pay IMF loans, ECB bonds maturing in July and growing state arrears, subject to lenders signing off on a review in its reform progress that includes changes to its tax and pension laws.
The proposed legislation would raise social security contributions, increase income tax for high earners and introduce a new national pension. It would also gradually phase out a top-up pension for low income earners.
Worn by years of austerity, Greeks fear that the new reforms will push the country further to the brink.
"We don't have food to eat and nobody asks us how we are," said shopkeeper Anna Papadopoulou, 74, who wept as she spoke.
Asked what she wanted to tell the Greek government, she said: "Wake up. We are dying."