Violence broke out after thousands of people gathered in Athens
Renewed clashes have broken out between protesters and police in Greece, in continuing unrest over the killing of a teenaged boy by police.
Demonstrators charged at riot police outside parliament, throwing fire bombs. Police responded with volleys of tear gas.
Twelve days after the police shooting, anger has combined with discontent in other parts of Greek society.
Protesters want the government to change social and economic policies.
Air traffic controllers are the latest public sector workers to go on strike.
An estimated 10,000 people joined a demonstration in Athens on Thursday that congregated outside a university and marched towards parliament, in anger at the shooting of a 15-year-old boy by a policeman on 6 December.
Banners castigated the government, which protesters accuse of failing the Greek people.
"Down with the government of blood, poverty and privatisations," one banner read, Reuters news agency reported.
The new violence erupted in the central square, site of the Greek parliament, as protesters threw petrol bombs at the building and attempted to burn down Athens' main Christmas tree.
The tree has already been replaced once after being torched during previous protests.
Some 70 people have been injured and about 400 have been detained during the protests. Hundreds of shops and banks have been vandalised and looted.
The policeman accused of shooting Alexandros Grigoropoulos, aged 15, has been charged with murder.
Some people were caught up in the protests as they shopped
Meanwhile all flights to and from Athens airport were halted for several hours on Thursday as air traffic controllers protested against government policies and demanded a pay rise.
It is part of an industrial action organised by the civil service trade union, ADEDY.
On Wednesday, protesters hung huge banners on the Acropolis, the ancient site that dominates Athens, calling for "resistance".
Conservative Prime Minister Costas Karamanlis has rejected calls to step down, despite growing public pressure.
But earlier this week he acknowledged some of the problems that had fuelled the anger of young people.
In a speech to parliamentary colleagues on Tuesday, he said "long-unresolved problems, such as the lack of meritocracy, corruption in everyday life and a sense of social injustice disappoint young people".
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