ATHENS - MASKED youths attacked the Greek riot police headquarters in Athens and protesters clashed with police in a northern city on Tuesday, in a revival of violence sparked by a teenager's shooting.

Police said a group of 30 youths threw petrol bombs and stones at the building, causing extensive damage to seven cars and a police bus parked outside. The central Athens building is also used by traffic police.

After a two-day lull, violence flared across Athens on on Tuesday.

Schoolchildren blocked streets and dozens of teenagers gathered outside the capital's main court complex and a maximum security prison - where some threw stones at police. Similar protests were planned in other parts of town later in the day.

Protesters have called for riot officers to be pulled off the streets, for police to be disarmed and for growing social inequality to be resolved.

The focus of the protests is now shifting to high school students, who were shocked by the death of 15-year-old Alexandros Grigoropoulos in a police shooting and have also voiced concerns at government economic, social and education policies.

Lessons have stopped at more than 100 secondary schools that are under occupation by students, according to the Education Ministry.

Scores of university buildings across Greece are also under occupation.

After the Dec 6 shooting, furious youths smashed and burnt hundreds of shops in Athens' main shopping area, and attacked riot police who responded with massive use of tear gas.

Dozens of people were injured in the rioting, while more than 300 people were arrested. The policeman accused of killing the teenager has been charged with murder and is being held pending trial.

In the northern port of Thessaloniki, riot police fired tear gas on Tuesday to disperse some 300 youths throwing fruit and stones outside the city's main court complex. The disturbance followed a court decision that found eight police officers guilty of abusing a student following riots two years ago.

The policemen received suspended sentences ranging from three years and three months for grievous bodily harm to 15 months for being an accessory to the abuse.

In a symbolic gesture meant to revive the festive mood in riot-shocked Athens, municipal authorities will light a large Christmas tree on Tuesday on central Syntagma Square - which has been at the centre of many of the past 11 days' protests. The tree replaces one burnt during last week's riots.

Overnight, unknown arsonists attacked three Athens banks with petrol bombs, causing extensive damage. There were no injuries or arrests. Every year, small anarchist groups carry out dozens of firebombings against government property, banks and diplomatic vehicles. -- AP