Reform bill would cut pensions by 10 per cent
The protesters from communist-affiliated party PAME hung out a huge banner reading: "We won't become slaves of the 21st century."
The bill gradually raises the retirement age to 67 years by 2022 and cuts pensions by 10 per cent for people who have retired but have yet to reach 67.
Athens needs to enact the reforms to successfully conclude a first review by the European Union and the International Monetary Fund to secure the next tranche of its 86-billion-euro international bailout and recapitalise its banks that have been hit hard by a deposit flight since December and the imposition of capital controls in late June.
"They want us to work until we get very old, without pensions and with no access to health and security," PAME said in a statement. "We should not let them destroy social security."
The bill will go to parliament's full session later on Thursday and a plenary vote is expected by Friday.
PAME and public sector union ADEDY will hold a rally in Athens outside parliament on Friday to coincide with the vote.
Greece has promised to present a comprehensive pension reform plan by December, while a panel on the pension system's viability is due to issue its report on Thursday, taking into account demographics and the deterioration in employment during years of crisis.
The left-wing government of Alexis Tsipras on Wednesday suspended plans to increase tax on rental incomes after a public outcry, saying it was still negotiating with international lenders on reforms ahead of the vote.
The government said on Thursday it had no other choice but to approve the bill to get its cash-strapped economy back on its feet.
"We have to vote the prior actions ... to conclude banks' recapitalisation now, to lift capital controls and to kick-start the economy," Education Minister Nikos Filis told ANT1 television.