Protesters in central Athens late Thursday threw stones at Greek riot police who retaliated with tear gas.
There were reports of looting as violence across Greece, triggered by a police shooting of a teenager, continued for a sixth day.
Rage over the killing turned into anger over corruption and economic hardship, shaking the conservative government of Prime Minister Costas Kamaranlis, which has a one-seat majority in parliament.
Mr. Karamanlis and opposition socialist leader George Papandreou have issued pleas for calm.
Scores of protesters and police in Athens and the northern city of Salonika have been injured since Saturday, when a 15-year-old boy was killed by police gunfire in Athens' central Exarchia district.
Protests have spread beyond Greece's borders, with demonstrations in other European cities, including Berlin, London, Copenhagen, Rome and The Hague.
Spanish youths attacked banks and police stations in Madrid and Barcelona. There were no reports of serious injuries.
The two Athens policemen involved in the shooting have been jailed pending trial. One faces murder charges while the second is accused as an accomplice.
The officers say they were only firing warning shots. Their lawyers say initial forensic analysis shows the boy was hit by a ricochet, and not a direct shot as some witnesses have said.
In a televised statement Wednesday, Prime Minister Karamanlis promised wide-ranging aid to help the more than 400 businesses damaged in the rioting.
Some information for this report was provided by AFP, AP and Reuters.