Hundreds of protesters have thrown stones and bottles at riot police outside the Greek parliament. Opposition leaders called for the government to resign as thousands of rioters went on the rampage across the country for a fourth day after the killing of a teenage boy. But Prime Minister Costas Karamanlis urged politicians to unite in the face of Greece's worst unrest for decades and warned rioters to expect no mercy. A Greek policeman fired into the air to disperse protesters near a police station in southern Athens after the funeral of Alexandros Grigoropoulos, who was shot dead by police on December 6. His death ignited anger over corruption scandals, a widening gap between rich and poor, and economic problems. More than 50 people have been injured and hundreds of buildings, including shops and other businesses, have been wrecked or burnt down in ten cities including Thessaloniki in the north and the tourist islands of Crete and Corfu. Arson and petrol bomb attacks have destroyed more than 130 shops in the capital Athens alone. Police have arrested 176 people during the protests and have used tear gas against stone-throwing youths but tried to avoid direct fighting, police officials say. Mr Karamanlis said: "We won't show any leniency. No one has the right to use this tragic incident as an excuse for acts of violence." The government faces a general strike on Wednesday over welfare reforms and economic policy. One policeman has been charged with murder over Grigoropoulos's shooting. Police said the officer fired three warning shots after their car was attacked by 30 youths on Saturday, but witnesses said he took aim at the teenager.