Hundreds of protesters clashed with police for a third day in several Greek cities in riots fuelled by growing economic hardship and triggered by the shooting of a teenager on Saturday night.
Police chased rioters with tear gas in Thessaloniki and clashes were reported in Crete, Corfu and elsewhere, while in Athens 13 police stations were damaged.
A 24-hour general strike is due on Wednesday against pension reforms and the government's economic policies, and many Greeks fear the demonstrations could last for days.
Some political analysts say prolonged violence and strikes could threaten the fragile conservative government.
More trouble was expected later on Monday. The Greek Communist Party has called a protest rally despite the arrest of two police officers for the 15-year-old boy's killing.
"We will not tolerate unacceptable and dangerous events prompted by the tragic incident," Prime Minister Costas Karamanlis said in a televised statement.
His ruling conservatives are lagging the socialist opposition in opinion polls and his party has only a razor thin majority in parliament.
"This comes at a very difficult moment for the government," said Anthony Livanios of pollster Alpha Metrics. "If this continues, it could have a devastating effect on the government and on stability."
Political analysts say an early election could be called next year.
WORST RIOTS FOR DECADES
More than 40 people have been injured and scores of businesses destroyed in Athens and Thessaloniki during Greece's worst rioting in decades. At least 35 people have been detained.
The streets of Thessaloniki filled with tear gas as police chased some 300 left-wing protestors, detaining two youths. Clashes were also reported on the islands of Crete and Corfu as well as the cities of Volos, Komotini and Chania.
In Athens, 13 police stations were damaged by demonstrators hurling stones and bottles, while students occupied university buildings, police said.
Demonstrators also occupied the Greek consulate in Berlin.
Cars and pedestrians returned to central Athens streets early on Monday as Greeks went back to work, but the mood was 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."
The shooting angered Greek youths, already resentful about a widening gap between rich and poor, and with hard times made worse by the global credit crisis. Violence at student rallies and fire bomb attacks by anarchists are common, especially in Athens' Exarchia district where the boy was shot.
Scores of shops and more than a dozen banks were torched in the capital's busiest commercial districts ahead of the busy Christmas period. Helicopters hovered overhead and tear gas choked the city. The mayor of Athens postponed the launch of holiday festivities.
Karamanlis announced financial support for those affected by the street violence, which showed no signs of abating. University professors started a three-day walkout on Monday and many school students stayed away from class in protest.
"He could've been our brother. He could've been our class mate, he could've been one of us," said Vangelis Spiratos, 13.
Two police officers have been charged over the shooting -- one with murder and the other as an accomplice. A police statement said one officer fired three shots after their car was attacked by 30 youths in Exarchia.
A police official said the officer had described firing warning shots, but witnesses told TV he took aim at the boy, identified as 15-year-old Alexandros Grigoropoulos.