Rioters have thrown fire bombs at police on a third day of protests in Greece against the fatal shooting dead of a 15-year-old boy.

The streets of the northern city of Thessaloniki were filled with teargas as 300 left-wing protesters fought with police, detaining two youths.

And more trouble is expected later in the day in Athens, where the Greek Communist Party has called a protest rally despite the arrest of two police officers for the boy's killing.

Cars and pedestrians have returned to Athens streets as Greeks went back to work, but the mood is tense. In the main shopping street, Ermou, a police team began to assess the damage.

"It is quiet now but I've never seen anything like it in my life," said Yiorgos Ganatsikos, 52, a kiosk owner. "I hope they don't continue. Otherwise, God help us."

At the weekend, thousands of youths took part in the violence across the country, burning scores of cars and shops in the worst protests to erupt in Greece in years.

In Thessaloniki, more than 1,000 protesters clashed with police, set fire to a bank and smashed several shops. Rioters also clashed with police in the western city of Patras.

About 200 protesters rioted outside police headquarters in Crete's second city of Chania, while on Corfu protesters smashed up four cars and two shops, and an 18-year-old woman was injured.

Pressure on the country's conservative government has shown no sign of easing. The socialist Pasok opposition, which is top in opinion polls, said Greeks must denounce the government.

"We must answer the government's policies en masse and peacefully," the Pasok youth branch said in a statement.

University professors, who had planned to join a nationwide workers' strike against pension reforms and economic policies on Wednesday, said they would now stage a three-day walkout. And blogs popular with students have urged them to stay away from school.

The shooting has touched a raw nerve among Greek youth, whose anger has been fanned by the growing gap between rich and poor in recent years. Violence at student rallies and fire bomb attacks by anarchist groups are common.

Two police officers have been charged over the shooting - one with premeditated murder and the other with abetting him.

A police statement said one officer fired three shots after their car was attacked by a group of 30 youths in Athens. A police official said the officer had described firing warning shots, but witnesses said he took aim at the boy, identified as Alexandros Grigoropoulos.

Just hours after his death, protesters clashed with police in Athens and the violence spread across the country.

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