Mobs threw stones and fire bombs at riot police as clashes broke out in Athens during a mass protest over budget cuts.
The rally of 30,000 in the capital was part of a general strike that crippled services and public transport around the country.
Police fired tear gas and stun grenades at protesters, blanketing parts of the city centre in choking smoke.
Thousands of peaceful demonstrators ran to side streets to take cover. A police officer was attacked and his uniform caught fire in the city's main Syntagma Square, and his motorcycle was burned.
At least two people were injured and another three arrested. One group of rioting youths smashed paving stones in front of the central Bank of Greece, but there were no immediate reports of any serious damage.
The rally had been calm before the clashes. Protesters chanting "Don't obey the rich - fight back!" marched to parliament as the city centre was heavily policed.
The demonstration was part of Greece's first major labour protest this year as Prime Minister George Papandreou faces international pressure to make more lasting cuts after the nation's debt-crippled economy was rescued from bankruptcy by the European Union and the International Monetary Fund.
The 24-hour strike halted trains, ferries and most public transport across the country, and led to the cancellation of more than 100 flights at Athens International Airport. The strike also closed the Acropolis and other major tourist sites.
State hospital doctors, ambulance drivers, pharmacists, lawyers and tax collectors joined school teachers, journalists and thousands of small businesses as more middle-class groups took part in the protest than have in the past. Athens' main shopping district was mostly empty, as many small business owners shuttered their stores.