Athens (AFP) - Greek riot police fired tear gas to disperse petrol bomb-throwing protesters in Athens Sunday as thousands took to the streets in anti-austerity demos ahead of a vote by lawmakers on a controversial tax and pensions overhaul.

Hooded youths lobbed flares and Molotov cocktails at officers who responded with volleys of tear gas, AFP reporters saw in clashes outside the parliament in the early evening, where some 10,000 protesters turned out to show their anger at the latest reforms demanded by Greece's creditors.

Police said more than 18,000 people rallied over the course of the day in the Greek capital, while around 8,000 turned out in the second city of Thessaloniki against the measures demanded by the EU and IMF and which the government is seeking to adopt ahead of a crunch meeting of eurozone creditors on Monday.

The reforms to be voted on later Sunday would reduce Greece's highest pension payouts, merge several pension funds, increase contributions and raise taxes for those on medium and high incomes.

The measures are part of an austerity package demanded by the European Union and International Monetary Fund in exchange for the next tranche of an 86-billion-euro ($95-billion) bailout agreed in July, the third for the debt-laden country since 2010.

- 'Tired and disappointed' -

Prime Minister Alexis Tsipras, who has said reform is needed to prevent the pension system collapsing in a few years, defended the changes in parliament late Sunday.

"The system requires root and branch reform that previous governments have not dared to undertake," he told lawmakers, adding that the reforms would not affect those on low incomes, something that was the result of "long and hard negotiations with creditors".

His left-wing Syriza party holds a slim majority with 153 seats in the 300-seat parliament.

Earlier, a sea of demonstrators marched through the streets waving banners with many of those taking part supporters of the communist-leaning PAME trade union.

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"Social security, public and compulsory for all. The plutocracy must pay," said one union banner.

Numbers were, however, significantly down on February protests when 40,000 people marched in Athens alone.

"People are tired and disappointed by the leftist government in power... the rallies have not had the scale we had expected," said Maria, a private sector employee in her fifties who claims to be owed 30,000 euros ($34,000) in back pay from her employer.

Finance Minister Euclid Tsakalotos has called on the eurozone to back the reforms, warning of a "failed state" if the Brussels talks run aground.

"The elements for closing the first review and providing debt relief are, I firmly believe, all there," according to a letter to the euro area's finance chiefs seen by AFP.

"Nobody should believe that another Greek crisis, leading perhaps to another failed state in the region, could be beneficial to anyone."

Greece's budget deficit has ballooned as it struggles to keep up with mammoth debt payments, which the IMF believes is unsustainable.

In its official agenda for Monday's meeting in Brussels, the Eurogroup said it would review the "progress achieved" by Greece as well as discussing "possible debt relief measures".

- Differences between creditors -

Greece was also in the grip Sunday of the third day of a general strike that has paralysed public transport across the country.

During the stoppages, the public sector has operated at a snail's pace, while most TV and radio stations have refused to air news bulletins.

Despite the pressure from the strikes, Employment Minister Georgios Katrougalos stood by the pensions overhaul, pointing to a funding shortfall of two billion euros.

"This reform should have been done decades ago," he said.

Ahead of the Brussels meeting, differences between the creditors themselves have emerged over extra reforms demanded by the IMF that could amount to another 3.6 billion euros of cuts by Greece.

IMF chief Christine Lagarde has warned that there were "significant gaps" in Greece's reform offers, while European Commission head Jean-Claude Juncker said Athens had "basically achieved" the objective of the measures required by creditors.

But both the EU and the IMF have agreed that debt relief could be considered.