ATHEN - UNREST in Greece, sparked by the fatal police shooting of a schoolboy, intensified on Wednesday with security forces battling protesters nationwide as a general strike crippled much of the country.

Hundreds of police again defended the Greek parliament building in Athens against thousands of demonstrators angered at the death of Alexis Grigoropoulos, who are now turning their sights on the right wing government of Costas Karamanlis.

With the country now in the fifth day of nationwide unrest over the death of the 15-year-old, the controversy heightened with initial results from a post-mortem on Grigoropoulos indicating he was killed by a bullet ricochet, legal sources said.

Demonstrators in central Athens hurled fire bombs, pavement slabs, tangerines, water bottles and other missiles in the latest street battles.

Riot police fired back tear gas as groups of youths chanted 'assassins' at them.

Athens was rocked by disturbances almost through the night and similar troubles were reported in the northern city of Salonika and Patras in the west.

Details of the autopsy on the boy's body were released just before the two police officers implicated in the shooting were to appear before an investigating magistrate on Wednesday.

According to forensic experts and independent experts acting for the Grigoropoulos family, the bullet 'is a bit deformed, which showed the bullet touched a hard surface' before entering the boy's chest.

Grigoropoulos was allegedly among youths who threw stones at a police car in a district of Athens that is known as a stronghold for political radicals.

Initial witness accounts said he was hit three times in the chest. His death set off nationwide unrest, including clashes at his funeral on Tuesday when riot police battled youths near the cemetery.

Anger at the police has been compounded by growing public frustration with the Karamanlis government. The general strike, called before the death of Grigoropoulos, brought public transport and most administrative offices to a halt. Airlines cancelled dozens of internal and international flights.

The opposition has demanded that the government resign and the Athens demonstrators marching on parliament chanted 'Sack Karamanlis,' and said he headed a 'killing state'.

In a televised address Tuesday, Mr Karamanlis blamed the disturbances on the 'enemies of democracy'.

He called for the general strike rallies to be cancelled 'as extremists could exploit them... to continue their violent and destructive activity'.

'This death was the catalyst for many grievances,' said 18-year-old farming student George Tzouvelekis, one of the protestors.

'Look how the banks are being attacked, because they have refused to lower interest rates amid the economic crisis... Everybody is fed up.'

Sixteen Greeks and 25 migrants were arrested during clashes and looting in central Athens during the night, police said.

The Athens Polytechnic and nearby Athens Law School have become a focus of the student protests. Student demonstrators holed up in the two universities fired petrol bombs at security forces who are surrounding the buildings.

A post office, a bank and a tourist office in the centre of Athens also damaged in the night-time violence.

In the city of Patras, at least 500 protesters laid siege to the police headquarters well into the night.

More than 80 shops and 14 banks were damaged during violence in Salonika where eight people were arrested.

Hundreds of protesters looted stores in Nea Smyrni, south of Athens, and residents said police fired their weapons into the air as they chased demonstrators. Police said they were investigating the allegations. -- AFP