Riots across Greece continued for the fourth straight day Tuesday as the country buried the 15-year-old whose shooting by police Saturday set off some of the worst violence in years.

Malcolm Brown from the town of Marazion in England was riding a bus near the Greek city of Piraeus on Monday when an explosion "rattled the bus windows," he wrote in an e-mail message to the BBC in which he described what he saw outside. "Hundreds of young people in the street scattered. Some were wearing hoods and masks. Sounded equivalent to a small stick of dynamite. … Several water cannons seen. Some streets closed off. Busloads of police. Several fire engines present."

This is the scenery that has dominated a dozen Greek cities since Saturday, when a teenager was shot and killed by police in Exarchia, a district of central Athens famous for violence and drug abuse. Two officers involved in the shooting have been arrested, yet the anger directed against Greek security authorities and the country's unpopular conservative government continues unabated.

The worst rioting hit Athens, where thousands of people, mostly teenagers, Monday evening torched large parts of the city center (including a large Christmas tree), damaging close to 200 hundred stores, banks and residential buildings, police said.

On Tuesday mourners clashed with police outside an Athens cemetery where the 15-year-old, Alexandros Grigoropoulos, was buried. In the city center, protesters clashed with police defending the Parliament building with tear gas. Demonstrators also protested at Greek consulates across Europe -- in Paris, London and Berlin.

Greek Prime Minister Kostas Karamanlis, whose party is clinging to a one-seat majority, urged the opposition to unite against the violence, vowing there would be no leniency in holding violent protesters accountable. Some 200 people have been arrested so far, and among them is a police officer who has been charged with murder over Grigoropoulos' shooting. Another officer has been charged as an accomplice to murder.

Police said the officer fired warning shots when his car was attacked by a group of rioters, but eyewitnesses have told the Greek press that the officer took aim.

Police repression has angered many young Greeks, who are also frustrated by poor funding for higher education, a lack of decently paid jobs, and corruption in business and politics.

"You come back from abroad with a master's degree and have to work for 600 euros ($780) a month in an unrelated field," Panayotis Adamopoulos, an electronics engineer, told the Financial Times. "How are you supposed to build a career?"

Yet while thousands of students are demonstrating peacefully, a group of youths has been extremely willing to use violence. The government has instructed police not to use firearms in order to avoid further killings. Confronted by angry shop owners demanding to know why their businesses had not been protected, security authorities said human lives were more important than glass. Yet the atmosphere remains heated, with massive brawls between rioters and police a regular sight on Monday and Tuesday.

Rioters, meanwhile, are hiding untouched at university campuses across Athens, which police have not been allowed to enter since 1973. In that year, tanks quashed a student uprising at Athens Polytechnic, killing at least 22 civilians.

The troublesome security all over Greece has led the opposition to argue that the country needs new leadership.

"The government is incapable" of stemming the crisis, Socialist leader George Papandreou told reporters Tuesday, calling for early elections and the resignation of the government.

As a 24-hour strike against pension and economic reforms is scheduled for Wednesday, observers fear the protests will go on for days.