Thousands of mourners gathered at the cemetery at the southern Athenian suburb of Paleo Faliro for the boy's funeral while just a few meters away 200 hooded youths clashed with police. Many of the hooded teenagers ran onto the streets and attacked banks, reports said.
Meanwhile, in downtown Athens, riot police attempted to fight back protestors as young as 10 years of age using tear gas to keep them from reaching the building.
Many demonstrators could be seen hurling firebombs and pieces of marble, and torching barricades in front of parliament in a violent press that threatens to topple the government.
In other parts of the city, students attacked four police stations in Nea Smyrni while elsewhere around the country, students clashed with police on the holiday island of Rhodes and in the northern city of Thessaloniki and the western city of Ioannina.
The funeral of 15-year-old Alexandros Grigoropoulos took place before nearly 3,000 mourners, many of them teenagers who gathered to pay their last respects and lay wreaths.
Grigoropoulos was allegedly among a group of youths that had thrown stones at a squad car in a district of Athens that is known as a radical stronghold. The policeman who fired the shots which killed him and his partner have been arrested. A coroner's report shows the boy was shot in the chest.
Teachers lead mass demonstration against government
Schools and universities across the country closed their doors for three days and hundreds of teachers, university lecturers and students rallied in central Athens.
Around 2,000 protesters, led by the OLME teachers' union, marched on parliament carrying a large banner reading "Assassins, the government is the culprit".
Greece braced for more widespread protests and possible violence following the funeral. The shooting was seen as the last straw by many young Greeks whose economic future is bleak in a country with a high unemployment rate and low wages.
Unemployment is pegged at over 7 per cent and nearly 20 per cent of Greeks live below the poverty line, earning less than 600 euros ($775) a month.
"Everyone appears to have let our children down. Students have become more hostile towards us and to figures of authority," said Christos Kittas, after resigning as the dean of Athens University after rioting spread to campuses.
Rioting, political pressure could bring down government
Public unrest has grown with the conservative government's austerity measures, with unions regularly demonstrating against privatizations, pension reforms and the cost of living, and this latest incident could topple the unpopular conservative government.
The main opposition Socialist party leader George Papandreou called on the government to resign in an effort to end the crisis.
"The government has lost public confidence," Papandreou told Pasok socialist party deputies. "The only thing it can give this country is to depart... to seek a public verdict so that the people can give a solution."
Three successive nights of rioting and looting have left hundreds of cars, stores and buildings charred and gutted across Greece and left many Athenians angry about the response of the government and police to the riots and their inability to stop the destruction.
Within hours of the shooting, riots erupted throughout Greece, starting from the capital Athens and quickly spreading to the northern port city of Thessaloniki, Volos, Thessalia and to the holiday islands of Corfu and Crete.
Almost every city across the country saw cars, buildings, banks and shops torched, looted or smashed.
Greek leaders' calls for calm go unheeded
Greece's president appealed for calm and Prime Minister Costas Karamanlis vowed to crack down on the unrest, but protesters again defied the government and there was new unrest in Athens and other cities.
Karamanlis called a crisis cabinet meeting on Monday night and held new meetings with President Karolos Papoulias and leaders of allied and opposition political parties on Tuesday.
"We will tolerate no leniency in the attribution of responsibility," the prime minister said after talks with the president.
Papoulias meanwhile appealed for calm, calling on Greeks to "honor Alexis' memory peacefully."
The government, already in trouble over the state of the economy and a series of political scandals, has been strongly criticized over the havoc.
"The whole country was delivered to chaos by an irresponsible government," the Eleftherotypia daily said in an editorial Tuesday.
The violence has showcased the organizational capacity of urban radicals and the failure of the government to crack down, critics said.
Public anger aimed at "outlaw" police
Many of the protesters say that the killing of Grigoropoulos was the latest example of police acting as if they were above the law.
"This is not just a random incident," said Magda, a 27-year-old hotel worker who was participating in a leftwing demonstration in Athens.
"It has to do with the role played by the security forces ... who enjoy impunity."
Lawyer Dimitris Beladis, who specializes in urban troubles, the killing "is the detonator of a sort of social explosion due to economic insecurity that affects many youths and those who are unemployed or badly paid."
Abroad, demonstrators attempted to take over the Greek embassies in London and Cyprus. In the German capital Berlin, protesters describing themselves as anarchists invaded the Greek consulate on Monday.
More than 60 people reportedly stormed the Greek embassy in Paris on Tuesday.
German police investigate sympathetic anarchists
German police were investigating damage done by anarchists in the eastern city of Dresden during an overnight protest linked to the Athens riots on Tuesday.
Between 20 and 30 black-clad sympathizers with the Greek anarchists first handed out leaflets.
Some of the group then tipped rubbish bins onto city streets, knocked over building-site fences and smashed a police-car windscreen. Police were seeking to identify them.