Firebombs are thrown in Athens' streets
Greek police have again clashed with demonstrators in Athens, two weeks after the fatal shooting of a teenager by police sparked nationwide unrest.
The violence followed a memorial gathering at the site where Alexandros Grigoropoulos died on 6 December.
Groups of demonstrators threw rocks and petrol bombs at police, who responded with volleys of tear gas.
Protests in Athens and other cities in the past fortnight over the police killing have often turned violent.
Greece's worst unrest in decades is fuelled by anger at high youth unemployment and unpopular government reforms.
The protests have caused hundreds of millions of euros in damage, rocking a conservative government that has a one-seat majority and trails the opposition in polls.
The protesters in Athens are using the National Technical University of Athens - known as the Polytechnic - as a base.
It is located near the central Athens neighbourhood of Exarchia where 15-year-old Alexandros Grigoropolous was killed.
Protesters have set up barricades outside the Athens Polytechnic
Security forces are not allowed on the grounds of the Polytechnic without permission from the university's administrators. No such authorisation has been given.
A group of youths attacked the Christmas tree in Syntagma Square in central Athens, tossing rubbish at it and attempting to set it on fire.
Riot police ringed the tree, which itself was a replacement for one burnt down at the height of the rioting, on 8 December.
The policeman accused of shooting Alexandros Grigoropoulos has been charged with murder.
Conservative Prime Minister Costas Karamanlis has rejected calls to step down, despite growing public pressure.
But he has acknowledged that "long-unresolved problems, such as the lack of meritocracy, corruption in everyday life and a sense of social injustice" are fuelling the anger of young people.