Written on 8th Dec:
On Saturday night, 6th Dec. a Greek 16-year-old was shot dead by a policeman. A police car with two policemen passed by and he threw stones at them. According to the police, he was part of a group of 25-30 people who attacked them. I would say that if there were 30 people, the policemen would have called for reincorcement, Instead, reportedly they parked the car and went back to arrest them. They threw a 'light bomb' and one of them shot the boy. (The policemen are now under arrest).
According to other reports, it was 5 boys initially and as the incident evolved more people gathered. The area where this happened is considered 'anarchist nest' - there are other, informal reports that it was the policemen who provoked the boys (ragrading them as 'anarcho-communists') and initiated the conflict.
It is also informally reported that the boy had nothing to do with political activity and was only considered an anarchist because he was in that area (not that being an anarchist would justify killing him, but maybe for the police it would).
These two policemen were part of a 'special force', with a history of violent treatment of demonstrators. The Greek police (regular and 'special') is also known for the right-wing to extreme-right wing beliefs and behaviours of its members (with few exceptions), some of them sympathisers with the 'ideals' of the 1967-1974 dictatorship in Greece, and the foreign-imposed post-Ottoman empire monarchy which was also abolished in 1974 with the fall of the dictatorship (http://en.wikipedia.org/wiki/Greek_Royal_Family#Downfall). As is often with the police, they have been more concerned with abusing immigrants. arrested demonstrators, Roma and other 'others' than with protecting citizens' rights.
Greece is in riot in the past 3 days (there are reports from literally every major city). Apart from hard-core fascists (unfortunately they exist), everybody is enraged with the unacceptable killing of the boy. Not everybody is sympathetic to the rioters as some are more concerned with material damages, the safety of people in the riot areas, or don't sympathise with the political beliefs of the anarchists who are reported as the sole source of the riots - despite the numerous demonstrations of pupils and students. the focus of the media is on the most spectacular, violent clashes, which are at a larger scale Greece has seen since 1973.
The police has made arrests and some fighting is reported between police and demonstrators. It appears that they are more reluctant in beating up people due to the incident of the boy's killing, but there are still reports of 'cornering' teenage demonstrators and beating them up instead of arresting them in Athens (http://papachatzis.tumblr.com/post/63589410/policeviolence). Apparently this happens mainly in Athens and not so much in other cities. Police are also generous with tear gas and other similar modern methods.
The CNN and BBC reports are at best unsympathetic (occasionally essentialist and stereotypical - see BBC commentary), but there is some factual information as well:
CNN report of some of the facts
Reporting and explanation from CNN