Riot police are attacked with firebombs thrown by rioters outside the National Technical University of Athens on Tuesday. (Petros Karadjias/Associated Press)Masked youths looted and rioted in Greek cities Tuesday night as the fallout from the police shooting of a teen continued for a fourth straight day.

The shooting has inflamed what had already been rampant discontent over the conservative government's economic policies.

While the violence died down as dawn approached, police braced for renewed friction Wednesday, when labour unions will participate in rallies and a nationwide strike to protest the government's policies.

Prime Minister Costas Karamanlis, facing mounting calls to resign amid nightly scenes of burning street barricades, looted stores and overturned cars, called on the unions to cancel Wednesday's rally in central Athens.

Hundreds of protesters gathered outside the Greek parliament on Tuesday as the funeral began for Alexandros Grigoropoulos, 15, who was shot on Saturday night.

More than 400 demonstrators threw stones and bottles at lines of riot police outside as party leaders met to discuss the growing crisis.

Youth, wearing handkerchiefs in attempts to ward off teargas being fired by riot police, chanted, "Let parliament burn."

"I am here because I have a feeling that something is happening, something is changing in society," Thodoros Adamopoulos, a private businessman attending the rally, told Reuters.

Opposition leader calls for early elections

Karamanlis appealed Tuesday for unity in the country, but socialist opposition leader George Papandreou has called for early elections, arguing the conservative government's inability to deal with the rioters has eroded public confidence.

"The government cannot handle this crisis and has lost the trust of the Greek people," Papandreou said.

A protester kicks a tear gas canister as riots continued Tuesday for a fourth day in Athens. (Petros Giannakouris/Associated Press)Karamanlis's governing conservatives have a majority of one seat in the 300-member parliament and have already recently faced one round of violent demonstrations — against their economic reforms. The latest widely televised scenes of destruction are likely to further undermine his government's razor-thin majority.

"This country does not have a government," Papandreou said. "This chaos ... is a result of the decisions and omissions made by a government that has become dangerous for the Greek people."

If the president agrees to call an election, it would be held within a month. The conservatives trail the socialists in recent opinion polls.

Observers have said the growing hostility among Greek youth is being fed by public discontent over low wages, frequent public corruption scandals and a strong historic distrust of government rooted in past political upheavals.

Some analysts have compared the riots to a 1973 student uprising, which helped topple a military junta.

Schools, universities closed

Greece has experienced three nights of widespread burning and looting that has gutted stores and buildings.

Karamanlis also called on unions to cancel a protest rally during a 24-hour strike scheduled for Wednesday so as to not further tensions in the country. But the request was rejected.

Schools and universities across Greece remain closed on Tuesday as more public demonstrations were planned to coincide with Grigoropoulos's funeral, held in a seaside suburb of Athens. More that 5,000 people gathered outside the church during the funeral, where the teenager's white coffin was covered in flowers.

Protesters were also moving toward the area where the funeral was held, Greek reporter Louis Economopoulous told CBC News.

Police were reporting Tuesday that groups of high school students were throwing rocks at police stations in western Athens.

Greek authorities have charged a 37-year-old police officer with murder in the shooting. A second officer has been charged as an accomplice in the shooting, which occurred in the volatile central Exarchia district. The police association has apologized to the teenager's family.

Mayhem nationwide on Monday

Karamanlis said there would be no leniency for the rioters.

"No one has the right to use this tragic incident as an alibi for actions of raw violence, for actions against innocent people, their property and society as a whole and against democracy," he said after an emergency meeting with the country's president, Karolos Papoulias.

Still, the clashes Tuesday were less severe than the rioting the previous three nights.

A man stands next to a burned bus ticket booth in central Athens on Tuesday. (Lefteris Pitarakis/Associated Press)Clouds of tear gas spread over at least 10 cities across the country where rioters continued their destructive marches on Monday.

Protests were reported in the tourist islands of Crete and Corfu, the northern city Thessaloniki and Athens, and spread to the Greek embassies in London, Berlin and Paris.

Beyond smashing windows, lighting trash cans on fire and tossing Molotov cocktails at police, rioters also set Athens' massive Christmas tree ablaze on Monday, and a luxury hotel in the city had to be evacuated.

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

With files from the Associated Press