Tens of thousands of Greek protesters angry about the government's austerity policies have demanded the heavily indebted country stop paying its creditors.
Many protesters carried signs and wore stickers reading "we don't owe, we won't sell, we won't pay" in the demonstration outside parliament.
The signs referred to planned privatisations that the government has agreed to speed up in order to make up for a shortfall in projected revenue.
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Greece was saved from default in May 2010 with a 110 billion ($A149.68 billion) bailout package of loans from the International Monetary Fund and European Union.
In return, it imposed strict austerity measures last year, including public salary cuts, pension reductions and broad tax hikes.
The measures have angered ordinary Greeks, sparking frequent protests.
In the latest one, demonstrators chanted "thieves, thieves" while pointing at the parliament building.
Several banners and placards called for a referendum on an updated bailout package the government is said to have agreed with its creditors.
The protest was the 12th in as many days, and was modelled after the demonstrations that took place in Spain last month.
Police officials said there were an estimated 60,000 protesters, while participants said there were many more.
There is no central organisation to the protests, which rely on scattered groups to spread the word through social media.
The crowds at recent protests have steered clear of partisan affiliations and at one, the protesters booed union members from the state electricity company who tried to raise their own banners.
In general, the protests have been peaceful.
In the northern city of Thessaloniki, an estimated 20,000 took part in a peaceful march. There was a minor scuffle when a group of anarchists went to a nearby book fair and tossed European Union publications to the street.
Crowds gathered in several other Greek cities, such as Patras in western Greece and Iraklio on the island of Crete.
The protesters vow to keep up the pressure indefinitely, although many have set June 25 as the crucial date, when parliament will vote on an updated fiscal austerity program covering 2012-2015.
The provisions of the program will be discussed at a Cabinet meeting on Monday.
