The country's worst riots in recent years began hours after a 15-year-old boy was shot Saturday night during a confrontation between police and youth in the Exarchia neighborhood of central Athens, a district of bars, bookshops and restaurants where clashes between far-left youth and the police have previously occurred.

As news of the death spread, hundreds of protesters took to the streets, burning scores of shops, cars and businesses while throwing fire bombs and stones at riot police, who countered with tear gas. At least six people were arrested in Athens for looting goods from the debris of destroyed department stores and boutiques.

The violence spread to other cities on Sunday, including Greece's second-largest city, Thessaloniki, as well as Chania on the island of Crete.

Stylianos Volirakos, an Athens police spokesman, said "dozens" of officers had been injured in their bid to seal off streets around Athens Polytechnic University, where rioters, hiding behind blazing trash bins and the university’s soaring gates, threw stones and fire bombs at security forces. It remained unclear Sunday night whether authorities would move to storm the state university, a move forbidden by Greek law after military tanks in 1973 rammed the gates of the school to quash a student uprising against the then-ruling military junta. At least 22 civilians died in that attack, which is marked every year by youth-led marches that occasionally turn violent.

Authorities fired several rounds of tear gas, which cloaked parts of Athens with plumes of acrid grey smoke. At least one apartment block was evacuated after masked youth torched a French car dealership and ensuing flames reached the balconies of residents, the private television station Alpha reported.

An Athens prosecutor charged two officers from an elite police corps with the shooting death of the 15-year-old, Andreas Grigoropoulos.

A 37-year-old officer who allegedly fired the shots was charged with manslaughter, while the other officer in the car was charged with abetting him, a statement from the prosecutor's office said. Agence France-Presse identified Epaminondas Korkoneas as the older officer and Vassilis Saraliotis as his partner.

According to the police, the two police officers had been patrolling Exarchia when their car was stopped by some 30 young men, many of them hurling stones, at about 9 p.m. Both officers left their car to confront the mob, "firing three shots that resulted in the death of the minor," according to the statement, even though witness accounts differ.

Private Greek media and a website popular among leftist youths, www.indymedia.org, said the teenager had been shot in the chest and died while being transferred to a local hospital.

Both officers were being detained at the nation's police headquarters in Athens, the police said.

Greece's prime minister, Costas Karamanlis, wrote a letter to the boy's parents expressing his sorrow over the shooting. "I know nothing can relieve your pain, but I assure you . . . the state will act, as it ought to, so that yesterday's tragedy won't be repeated," he wrote.

"It is inconceivable for there not to be punishment when a person, let alone a minor, loses their life," Interior Minister Prokopis Pavlopoulos said at a Saturday news conference. "The loss of life is something that is inconceivable in a democracy."

As government officials quickly moved to condemn the shooting, thousands took the streets in protest. A march on Sunday in downtown Athens with some 3,000 demonstrators was peaceful until it was interrupted by youths throwing more rocks and handmade bombs, and police shot tear gas in an attempt to disperse the crowd. Private television networks broke into scheduled programming to broadcast the street fighting. Young men were seen smashing storefronts, targeting banks and burning dozens of refuse containers and cars along the meandering streets of Athens' high-end commercial district.

Mr. Pavlopoulos, who offered to resign early Sunday, called for restraint. His resignation was not accepted by the prime minister.

"People have the right to protest and will do so," Mr. Pavlopoulos said, "but while the pain and grief caused by the minor's death is understandable, no outrage...can lead to the violence and destruction of private property that was witnessed."

Sharon Otterman contributed reporting from New York.