Demonstrators fought security forces outside the country's biggest prison and a university in central Athens while police said groups of youths attacked stores in several districts or blocked main roads.

Formal voluntary homicide charges against the police officer accused of shooting 15-year-old Andreas Grigoropoulos failed to stem public anger and Greek embassies in other countries have also become a target for protests.

Under fire prime minister Costas Karamanlis flew to a European Union summit in Brussels.

A clash at Korydallos prison in a western Athens suburb blew up after protesters threw rocks and other missiles at police who fired tear gas to force the protesters back.

Demonstrators later staged a sitdown protest in front of the prison amid more confrontations with security forces.

Police said there was also unrest at the Athens agriculture university, which has been occupied by students, and that rampaging youths attacked stores in the upmarket Nea Smyrni and Galatsi districts of the capital.

More than 100 schools and some 15 university campuses remain occupied by youth demonstrators in Athens and the second city of Thessaloniki, with student groups having announced another rally for Friday.

Slogans such as "state killers," "murderers, you will pay," "democracy gives arms, cops assassinate" and "Silence only shows complicity" are splashed across banners hanging outside the Athens Polytechnic.

Around 20 youths threw stones at police who responded with tear gas on the sidelines of a leftist demonstration of a few hundred people who marched to the parliament.

A similar demonstration by 1,000 people in the northern port city of Thessaloniki ended peacefully.

Six days of unrest in cities across Greece since Grigoropoulos was fatally shot in Athens have caused dozens of injuries and left hundreds of banks, stores and public buildings destroyed, badly damaged by fire, or looted.

Athens police officer Epaminondas Korkoneas was charged Wednesday with voluntary homicide -- which under Greek law does not necessarily involve premeditation. Korkoneas has claimed self defence with ballistics analysis indicating a ricochet bullet hit the schoolboy, lawyers said.

His partner, Vassilios Saraliotis, 31, was charged with being an accomplice and will also remain in custody.

Demonstrators and left wing unions have sought to focus the public anger against the right wing government, whose popularity has plummeted in recent months because of the economic crisis and a series of political scandals.

A general strike on Wednesday brought much of the country to a standstill and badly disrupted flights in and out of Greece.

The socialist opposition has also stepped up calls for Karamanlis to quit and call new elections, following the worst unrest Greece has seen since the end of a military dictatorship in 1974.

Karamanlis now faces intense domestic political pressure, with just a single seat majority in parliament.

"We need a new direction particularly now that there's an international economic crisis where we need a stable government which has the confidence of the people and which has credibility with the people of Greece," socialist leader George Papandreou told French TV.

Korkoneas is accused of killing Grigoropoulos during a clash with around 30 youths in the Exarchia district of Athens. His lawyer said Korkoneas claims self defence saying the group threw firebombs and other objects while shouting that they "were going to kill them."

The crisis has crossed borders, with Turkish left-wing protestors daubing red paint over the Greek consulate in Istanbul, and Greek embassies in Moscow and Rome also targets for firebombers.

In Spain, 11 demonstrators were arrested and several police officers injured during clashes in Madrid and Barcelona, while 32 people were arrested in Copenhagen when their protest in support of Greek rioters turned violent, police said.