Gangs of young people set fire to the Christmas tree in Athens' central Syntagma Square on Monday, after the fatal police shooting of a teenager on the weekend. (Thanassis Stavrakis/Associated Press)Massive riots, sparked by the fatal police shooting of a teenager, continued to tear through Greece on Monday as thousands of youths smashed windows and torched buildings in Athens and Thessaloniki.

Clouds of tear gas spread over Athens as rioters continued their destructive marches, which included torching the city's massive Christmas tree in central Syntagma Square, the Associated Press reported. Some protesters posed for photos in front of the blaze. Others sang the Greek version of O Christmas Tree.

Guests had to be evacuated from the luxury hotel Athens Plaza after windows were smashed and the lobby filled with tear gas or smoke. Protesters reportedly launched attacks on banks and government ministries as well, and looted a store selling hunting weapons and swords.

"We are not counting any more…The incidents cannot be counted," one Athens fire brigade officer told Reuters.

Protests were reported in 10 cities across the country, including the tourist islands of Crete and Corfu and the northern city Thessaloniki, where rioters smashed buildings and threw Molotov cocktails.

Greek authorities have charged a 37-year-old police officer with murder in the shooting of 15-year-old Alexandros Grigoropoulos in Athens on Saturday night.

Police apologize

A second officer has been charged as an accomplice in the shooting, which occurred in the volatile central Exarchia district. The police association has also apologized to the teenager's family.

Schools were set to be shut Tuesday in mourning, while staff at universities declared a three-day strike.

Monday's violence came despite appeals for calm by Greek Prime Minister Costas Karamanlis.

Rioters throw stones at police during a student protest in the northern Greek port city of Thessaloniki on Monday. (Nikolas Giakoumidis/Associated Press)

"All the dangerous and unacceptable events that occurred because of the emotions that followed the tragic incident cannot and will not be tolerated," Karamanlis said in a live televised address. "The state will protect society."

The Athens shooting resulted in thousands of civilians clashing with police and rampaging through the capital and other Greek cities over the weekend. Scores of businesses were destroyed and dozens of people were injured in the riots.

The exact circumstances of Grigoropoulos's death are not yet clear. He was shot in the left side of the chest and died before an ambulance transported him to a state hospital, Health Ministry officials said.

The two policemen involved in the shooting said they had come under attack by a group of about 30 youths, and that three warning shots and a stun grenade were fired when they sought out the group a few minutes later.

But witnesses have disputed the officers' accounts and allege the police intended to shoot the youths.

Autopsy results that will reveal if the deadly shot was a ricocheted bullet or a direct shot are expected this week.

A riot police officer passes a tear gas canister during a student protest in Thessaloniki on Monday. (Nikolas Giakoumidis/Associated Press)

As night fell Monday, gangs torched stores and smashed bank branches in the city centre, while clashes broke out in the streets near parliament.

Gangs rampaged through Athens's upscale Kolonaki district, torching cars and smashing windows, including those of the American Hellenic Union, an educational charity, as they went. Masked youths tried to smash a fire engine attempting to put out a fire.

Violence often breaks out between riot police and Greece's self-styled anarchist movement, which traces its roots to the 1967-74 resistance to the country's military dictatorship. The general anti-capitalist, anti-establishment principles of the anarchist movement still attract youth to the group today.

Karamanlis's governing conservatives have a slim majority of one seat in the 300-member Parliament and have already recently faced one round of violent demonstrations — against their economic reforms.

The government is growing increasingly unpopular while the opposition Socialists are ahead in the opinion polls.

Police used tear gas in the streets of Thessaloniki on Monday to push back the protesters, who were throwing fire bombs, overturning trash cans and setting fires on one of the city's major streets, according to Reuters.

Reports indicated that two youth had been detained in Monday's riots.

In nearby Veria, police were also using tear gas on about 400 high school students who began throwing rocks on Monday morning.

Demonstrations have also reportedly turned violent in the small central Greek town of Trikala.

The Associated Press reported that one policeman was injured in Trikala and a nearby rail route was blocked.

In Athens, gangs of youth throwing rocks were also sporadically attacking police stations on Monday. The Interior Ministry was also stoned and youth clashed with riot police outside the parliament buildings.

Protests in Berlin, London

Masked protesters shout slogans from the Greek Consulate in Berlin on Monday. (Herbert Knosowski/Associated Press)"Cops! Pigs! Murderers!" the protesters bellowed at the police in Athens.

In Chania, high school students also took to the streets, throwing broken chairs, rocks and wood at riot police.

Meanwhile in Berlin, 15 youth occupied the Greek Consulate and in London demonstrators pulled down the Greek flag at the country's embassy and raised a red-and-black anarchist banner.

Police said 14 people were arrested in Athens over the weekend for public disturbance offences and looting.

Health officials said hospitals treated 29 people with minor injuries sustained during Sunday's violence.

With files from the Associated Press