Alexis Tsipras intends to put the reform bill to parliament early next month.
Leftwing Greek Prime Minister Alexis Tsipras, at a rally Sunday to mark a year in power, stood by a vow to overhaul the country's pension system despite fierce opposition.
Pension reform "must go ahead, it's necessary," Tsipras said in a speech before around 4,000 supporters in an Athens stadium.
"The dilemma is about reforming the system or letting it collapse," he said.
Tsipras intends to put the reform bill to parliament early next month.
But he faces stiff opposition from leftists in his Syriza party and other and social groups which will be hit by bigger contributions to the pension pot.
In his speech, Tsipras promised however to "exhaust every avenue of dialogue and flexibility" to help those affected most -- small companies, the self-employed and farmers -- and said the changes would be "fairer".
Tsipras also reiterated his commitment not to reduce pensions from their current levels, saying they had "already been cut by 40 percent" by previous governments under pressure from the European Union (EU) and International Monetary Fund (IMF), Greece's main creditors.
Syriza rode to power on January 25 2015 in an election where it won 36.3 percent of the vote, thanks to a pledge to renegotiate the terms of Greece's international bailout.
There then followed months of tense negotiations in Brussels, and particular with Germany.
Eventually, Greece accepted its third bailout deal in five years, but had to swallow even harsher reforms.
Tsipras was re-elected on September 20, and formed a second straight coalition with the nationalist Independent Greeks (ANEL) party.
The premier on Sunday lashed the rightwing opposition, which according to the opinion polls is enjoying a resurgence in power, and claimed that his struggle with Brussels had wrought lasting change in the EU.
"Europe is no longer the same," he said, referring to an anti-austerity swing in sentiment in Portugal, Spain and Italy.