Lawmakers passed the plan late Sunday after protesters burned buildings, smashed windows and pelted police with rocks and gasoline bombs. Police attacked the rioters with tear gas.
Around 100,000 protesters marched through Athens and the second largest city, Thessaloniki.
At least 10 buildings in Athens were set on fire, including movie theaters, cafes and banks.
Prime Minister Lucas Papademos condemned the violence, saying it has no place in a democracy.
The marchers ranged from young men with masks and shaved heads to families and the elderly. Many Greeks say they already have sacrificed enough as the government strives to emerge from its huge debt and avoid bankruptcy.
The Greek Cabinet approved a new austerity package on Friday before sending it to parliament. Thousands of jobs would be eliminated, the minimum wage would be cut, and the pension system would be reformed.
The European Union and International Monetary Fund demanded thar all Greek political parties agree on the austerity plan before final approval of a $172 billion bailout loan.
Greece faces bankruptcy if it cannot pay investors $19 billion in debt when government bonds come due next month. The government is negotiating a bond swap with the creditors that would save the country billions of dollars.
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