Greece faced its most extensive and violent riots in a quarter century Tuesday, as masked youths and looters marauded through cities for the fourth night in a rage sparked by a teenager's death in a police shooting. The nightly scenes of burning street barricades, looted stores and overturned cars across the country has left Greece's increasingly unpopular conservative government reeling, with calls mounting for Prime Minister Costas Karamanlis to resign. On Tuesday, police fired tear gas at protesters after the dead teenager, 15-year-old Alexandros Grigoropoulos, was buried in an Athens funeral attended by about 6,000 people. Riots began anew in several cities, while Greek television stations broadcast footage showing riot police using pistols to fire warning shots into the air. Opposition Socialist leader George Papandreou called for early elections, charging that the conservatives are 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 best thing it can do is resign and let the people find a solution ... we will protect the public." Many protesters, while not voicing particular policy goals, also 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 after a protest in Athens. "This government wants the poor to pay for all the country's problems - never the rich - and they keep those who protest in line using police oppression." But Karamanlis, who narrowly won re-election a year ago, has ignored the calls. He trails the Socialists in recent opinion polls and would struggle to win a general ballot now. But while the government has only a single seat majority in the 300-member Parliament, it is unlikely any deputy would risk their political career to bringing down a government during civil unrest. A poll released Tuesday gave the Socialists a 4.8 percent lead over Karamanlis' conservatives. It gave no margin of error. Still, the rioting - which has rocked cities from Thessaloniki in the north to the holiday island of Corfu and Crete in the south - threatens the 52-year-old prime minister, who has already faced months of growing dissatisfaction over economic and social reforms at a time of deep anxiety over growing economic gloom. More protests are expected Wednesday. Senior Socialist official Christos Protopappas blamed underlying social inequalities for the violence, saying the government's policies have exacerbated the gap between rich and poor. "If there is no change in policies, I fear that what will happen in six months or one year will be much worse," he said. And the protests are sure to further undermine the government's ability to push through reforms. "This reaction will register as major discontent in the next public opinion polls, which will hobble the government's effectiveness," political analyst Anthony Livanios told The Associated Press. "Whenever the government tries to pass certain policies, Greek society will react - while the level of parliamentary opposition will increase." But even if the opposition Socialists were to come to power, they would find themselves faced with implementing many of the same unpopular reforms of Greece's economy and its fractured pension system. Karamanlis himself has called for unity, saying that "those who cause violence and vandalism are enemies of democracy." Although it was a student revolt that eventually overthrew Greece's seven-year military dictatorship in 1974, it is rare for street-level violence or revolt to topple a Western European government, particularly one that has been democratically elected. The fallout from the riots, which has seen police stations nationwide come under attack by rock- and Molotov cocktail-throwing youths, could be far-reaching. "At a time when reform and counterattack is required, in light of the global economic crisis, the winds of destruction are blowing through our city," Athens Mayor Nikitas Kaklamanis said. "The destabilization of the market and the global defamation of Athens, not simply as a tourist destination, is, likely, the least significant predicament which the average citizen ... faces on the eve of Christmas," he said in a statement. Rights group Amnesty International accused Greek police of heavy-handed tactics against protesters. The human rights watchdog said its members in Greece reported that riot police "engaged in punitive violence against peaceful demonstrators" instead of focusing on rioters. On Tuesday night, rioters hurled rocks and Molotov cocktails at police stations in the western city of Patras, as well as on the island of Corfu and the northern town of Veria. "We find ourselves before a social uprising of youth," said Alekos Alavanos of the Coalition of the Left party, which has often been accused of being too soft on rioting youths. Tension between security forces and leftist groups is deeply rooted in Greece, dating back to the uprising against the country's 1967-1974 junta. The groups have now evolved into various factions that claim to fight trends ranging from globalization to the growth of police surveillance cameras. Their impact is usually limited to graffiti and late-night firebomb attacks on targets such as stores and cash machines. But the latest riots have moved far beyond the small anti-establishment groups to become a siege on Karamanlis' government. Teenagers and university students have joined self-styled anarchists in much of the rioting and destruction. "This was an emotional reaction after public opinion was outraged by the unfortunate event of the teenager's killing," said Livanios. "Clearly, during very negative economic conditions people with very low incomes and jobless people who can see no future for themselves became part of this social reaction."