About 30,000 people protested in Athens' central Syntagma square on Sunday evening against the government's tough economic austerity policies.

The demonstration, larger than many others that have taken place during Greece's economic crisis, appeared to be the first that resulted from spontaneous calls over social media sites such as Facebook. Many others have been organized by unions or political factions.

Pointing at Parliament and its 300 legislators, many of Sunday's protesters chanted "Thieves! Thieves!" Others shouted slogans against Prime Minister George Papandreou and the International Monetary Fund.

No violence was reported and few police were on the scene.

Papandreou, buffeted by negative polls and protests such as these, has vowed to continue his fiscal reforms.

On Sunday, he warned against "sirens" calling for Greece to default on its debt. Speaking to an audience of local officials in western Greece, Papandreou said it is "foreign taxpayers' money" that has allowed his nation to continue paying public sector salaries and pensions.

Reforms are painful, but they are starting to pay off and the economy will return to growth in 2012, he said.

Two polls published earlier Sunday showed the governing Socialists neck and neck with opposition conservatives. In one poll, the conservatives led for the first time since they were trounced at elections 20 months ago.

Debt inspectors from the European Union, the IMF and the European Central Bank are in Athens to monitor progress on last year's euro110 billion ($155.8 billion) bailout loan deal for Greece, which remains shut out of the bond markets and is facing a potential financing gap in 2012. They will determine whether Athens has met criteria for a June loan installment, worth euro12 billion and vital to avoid a summer default.

But the austerity measures have significantly eroded the incomes of many Greeks, causing widespread anger and further depressing an already shrinking economy. Greece's economic output dropped 4.5 percent in 2010 and is expected to shrink a further 2.7 percent this year.

Popular anger at the measures has made calls for Greece to default popular, despite warnings by ministers that the country cannot afford such an option.