ATHENS - GREEK police fought street battles on Friday with youths who hurled petrol bombs and stones amid new demonstrations over the police killing of a schoolboy.

After one of the calmest nights since the death of Alexis Grigoropoulos last Saturday sparked nationwide riots, police stormed about 100 youths on the sidelines of a demonstration, seizing a number of them and wrestling them to the ground.

The youths had thrown firebombs and stones at police who fired back tear gas before the protest rally started a march toward the Greek parliament.

Authorities remained on alert with scores of university campuses and schools across Athens and Greece's second city of Thessaloniki still occupied.

Militant youths have staged attacks from behind university walls throughout the violent protests, taking advantage of a law that prevents police from entering educational establishments.

The only violence reported overnight saw rocks thrown at a sports club headed by Antenna television owner Minos Kyriakou, who is also the chairman of the Greek Olympic Committee, police said.

A protest march on parliament in Athens on Thursday evening ended in minor clashes between youths and police while another demonstration drawing over 1,000 protesters in Thessaloniki ended peacefully.

The nationwide unrest has left hundreds of banks, stores and public buildings destroyed, badly damaged by fire or looted. Prime Minister Costas Karamanlis has offered cash help to businesses.

But Mr Karamanlis, whose parliamentary majority consists of just one deputy, already shaken by corruption scandals and opposition to unpopular reforms, now faces a new political crisis.

Greece's socialist opposition has stepped up calls for the premier - in Brussels for an EU summit - to call new elections, amid the worst unrest Greece has seen since a military dictatorship ended in 1974.

The crisis has crossed borders with Greek embassies in Russia, Italy, Spain and Denmark and other countries becoming the target for sometimes violent protests.

In Germany, demonstrators threw stones, bottles and paint at police cars during a protest rally in Frankfurt on Thursday night. Two officials were injured and police said eight arrests were made.

Greek protesters have said they are striking out against police repression, corrupt politicians and a social system that offers little hope, while the government has blamed the violence on loosely organised self-styled anarchists.

Slogans such as 'state killers,' "murderers, you will pay,' "democracy gives arms, cops assassinate' and 'silence only shows complicity' are splashed across banners hanging outside the Athens Polytechnic at the centre of the youth movement.

Despite the two officers implicated in the schoolboy's death being placed in pre-trial detention, fresh uproar was caused this week when their lawyer said the death 'was sadly brought about by an act of God'.

The officer who shot Grigoropoulos says he was trying to defend himself from a gang of youths and killed the boy by accident due to a bullet ricochet.

A ballistics report, which is said to confirm the officer's deposition, has yet to be officially released. -- AFP