The fatal shooting of a teenager has plunged Greece into chaos after ferocious rioting which saw police and protestors battling each other on the streets.

Around 150 masked youths, who barricaded themselves into the Polytechnic University in Athens, attacked security forces on Monday morning with firebombs and stones. Police, who had exhausted their supplies of tear gas, defended themselves by throwing stones, according to media reports.

Hundreds of students who had taken part in the attacks sought refuge in the Polytechnic and Economic universities, where police cannot enter under Greek law.

The chaos also continued in other areas. Houses and shops were set on fire and smoke bombs and stones flew through the air.

So far, the riots have caused 40 injuries, with one woman who was among the most seriously-hurt taken to hospital with head wounds, and have seen 13 arrests.

How did the riots - Greece's worst in decades - come about? The catalyst was the death of a 15-year-old schoolboy on Saturday, who was apparently shot dead by police.

A blurry video on internet site YouTube partly shows what happened: Two shots can be heard and then two men are seen running away from the scene of the crime.

Eyewitnesses claimed that the two men were policemen, who offered no help to the dying boy.

The police have said that a police patrol car was attacked by 30 youths. The two men fled and in doing so had let off a ‘flashbang’ stun grenade and fired warning shots.

A bullet - which the police saw was a ricochet - hit 15-year-old Andreas Grigoropoulos in the chest.

Both policemen and their supervisors have been suspended. The man who fired the shot has been charged with manslaughter, his colleague with complicity.

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The death has had dramatic consequences as the confrontations between masked demonstrators and police took place across the country.

During the chaos on Saturday night, around 31 shops, a new bank and 25 cars in the Athens shopping area of Exarchia were burned or destroyed. Six police cars were also targeted, according to authorities.

A three-storey shopping centre in the main shopping street Ermou went up in flames and burning barricades were set up on the streets.

Amongst the shops hit were international chains Benetton and H&M. Other small shops were also destroyed in the onslaught, but smaller restaurants in the central Syntagma Square were unaffected.

Riots continued on Sunday as more than 1,000 people gathered for a protest march against the Athens police which descended into chaos amid more firebombs. Masked youths belonging to an anti-authoritarian group broke shop windows and car windscreens and set rubbish bins on fire as a barricade. Banks and shops also went up in flames.

The situation has expanded into other areas - demonstrators also took to the streets in Salonki in northern Greece, Thessaloniki and in Herakleion in Crete.

Will the riots have political consequences?

Interior Minister Prokopis Pavlopoulos and his deputy have already offered to resign, although Prime Minister Konstantinos Karamanlis has so far refused to do so.