Riots have swept through more than 10 Greek cities since the December 6 killing of Alexandros Grigoropoulos, leaving a trail of destruction as youths looted shops and set fire to hundreds of cars, banks and businesses.

Demonstrators gathered outside the capital's police headquarters, prison and a university in a silent protest blocking streets. Around 100 people also held rallies near the country's education ministry.

Although protesters set fire to three banks and several shops in Athens on Saturday, police said Sunday was calm. Meanwhile, the public unrest spread last week to other European cities, as hundreds of protesters were detained in Spain, France and Denmark in similar incidents.

An opinion poll published by the Ethnos newspaper on Sunday said 83 % of Greeks were unhappy with the government's measures to stop the violence.

Two police officers have been detained over the killing. One of them, Epaminondas Korkoneas, who claims he fired warning shots in self-defense which ricocheted, was charged with murder and the illegal use of his weapon. The second officer, Vassilios Saraliotis, was charged with aiding and betting Korkoneas.