Athens Greece ? Masked youths and looters marauded through Greek cities for a fourth night Tuesday, in an explosion of rage triggered by the police shooting of a teenager that has unleashed the most violent riots in a quarter century.

The nightly scenes of burning street barricades, looted stores and overturned cars have threatened to topple the country’s increasingly unpopular conservative government, which faces mounting calls for Prime Minister Costas Karamanlis to resign.

Police fired tear gas at protesters following the funeral of 15-year-old Alexandros Grigoropoulos, who was laid to rest in an Athens burial attended by about 6,000 people.

Violence calmed before dawn today, but police were braced for more trouble later in the day when labor unions planned rallies during a nationwide strike called to protest the government’s economic policies.

The rioting — which has engulfed cities from Thessaloniki in the north to the holiday island of Corfu and Crete in the south — threatens the 52-year-old Karamanlis, who already faced growing dissatisfaction over financial and social reforms at a time of deep anxiety over growing economic gloom.

Opposition Socialist leader George Papandreou called for early elections, charging the conservatives were incapable of defending the public from rioters.

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

The call was echoed by protesters, who, though they have not voiced any particular policy goals, say they want Karamanlis out.

“It’s very simple — we want the government to fall. This boy’s death was the last straw for us,” Petros Constantinou, an organizer with the Socialist Workers Party, said.