Athens  - Ashes, black fumes, twisted debris and broken glass were scattered Tuesday throughout much of what once resembled Athens after the capital and dozens of other Greek cities were crippled by arson and looting for a fourth straight day.

Described as the country's worst riots in decades, tear gas and thick, black smoke engulfed central Athens for hours as students bent on arson set fire to cars, banks, shops and several government buildings.

The offices of state airline Olympic Airways, the Foreign Ministry across from Parliament, a luxury department store and Greeces main law school were torched, burning out of control as frantic firefighters attempted to extinguish the flames.

By early Tuesday, hundreds of students taking part in the riots sought shelter in the Polytechnic University, which is off limits to police under Greek law, reports said. Youths could be seen setting fire to barricades and hurling fire-bombs outside the university at riot police, who responded with tear gas.

The Polytechnic University was the scene of a major student uprising during the military dictatorship that ruled Greece from 1967-74.

Wide-spread riots first erupted late Saturday, shortly after a teenager was shot to death by a police officer in Athens' bohemian Exarchia district.

Although the circumstances surrounding the shooting remain unclear, two officers have been arrested and charged with manslaughter. A coroners report showed that the boy was shot in the chest.

The teenager's funeral is scheduled for Tuesday afternoon in the seaside district of Paleo Falio, just south of the city centre, and universities have been ordered closed for three days.

In a show of solidarity, high school and university students on Monday joined self-styled anarchists in the riots, which have been raging out of control since the first news of the police shooting.

Protestors used firebombs, chunks of marble, stones and iron rods in their attacks against police throughout the day.

As dusk fell on Athens, hooded youths could be seen setting fire to the city's massive Christmas tree and festive village and were reportedly making their way toward heavily guarded Parliament buildings.

Rioters, mostly in their teens, clearly had control of the city by late Monday and were looting stores and taking anything that could be used as a weapon.

Dozens of shops, parked cars, banks and even luxury hotels had windows smashed as youths fought with riot police, who retaliated by beating protestors with batons while arresting others.

Frightened pedestrians and tourists, many suffering breathing problems from the tear gas, ran for shelter.

The luxury hotel Athens Plaza in central Sytagma Square had all of its guests evacuated, reports said.

Earlier, more than 10,000 protestors from the Communist Party and other left-wing groups marched through the centre of Athens chanting anti-government slogans.

Prime Minister Costas Karamanlis again appealed for calm after an emergency meeting Monday night with his ministers and top security officials, trying to find a way to break the chain of violence and destruction. The interior minister emerged from the meeting saying the massive protests were unacceptable but insisted that police were doing all they could to protect lives and property.

With anger over economic hardships brewing for months and with many youths uncertain of their futures, the latest incident over the teenager's slaying could topple the unpopular conservative government.

Thousands of students, armed with fire bombs and stones, clashed with police and smashed storefronts in the northern port city of Thessaloniki, and similar violence was reported in at least in eight other cities across Greece.

From the northern city of Veria to the central city of Trikala, rioters committed arsons and destroyed businesses, cars, banks and public buildings. A group of protestors even took over a television station in the western port city of Patras.

On Crete and the island of Corfu, massive riots took place with gangs of students torching banks and police stations.

Fire departments in both major Greek cities tried frantically to extinguish the flames, and in Athens firefighters were nearly burned to death when their truck was set on fire by protestors.

Hundreds of shopkeepers watched in dismay as their businesses were destroyed ahead of the Christmas shopping season, which retailers had hoped would make up for the financial crisis that has hit Greece.

The damage nationwide is estimated in the millions of euros.

Police have detained more than 50 people and 100 more were reported injured.

The head of the journalist federation, Panos Sombolos, called it the worst rioting in Greece in his 30 years as a journalist.

Abroad, demonstrators attempted to take over the Greek embassies in London, Berlin and Cyprus. At the Greek embassy in London, protestors took down the Greek flag and raised a black and red anarchist flag in its place.

Public unrest has grown with the conservative government's austerity measures, with unions regularly demonstrating against privatizations, pension reforms and the cost of living. Twenty per cent of Greeks live below the poverty line.

"Enough of this government, which does not understand the problems of this country," main opposition Socialist leader George Papandreou said.

A new 24-hour general strike was called for Wednesday, and analysts predicted a new wave of riots to strike Greece. (dpa)