By Daniel Flynn and Dina Kyriakidou
ATHENS (Reuters) - Greece braced for a third day of demonstrations on Monday after the fatal shooting by police of a teenager triggered underlying anger over the Conservative government's economic policies and the worst rioting in decades.
Thousands clashed with police and rampaged through Athens and other cities this weekend, destroying scores of businesses, injuring dozens and piling pressure on the conservative government, whose ratings have already been hit by a slowdown.
"Athens and Thessaloniki under siege" said daily Eleftheros Typos on its front page, while Apogevmatini newspaper headlined: "48 hours of horror."
Despite the arrest of two police officers for the killing of the 15-year-old boy, the Greek Communist Party announced a mass rally in central Athens for Monday evening and the socialist PASOK opposition, which has taken the lead in opinion polls recently, called for peaceful mass demonstrations.
Cars and pedestrians returned to Athens streets as Greeks went back to work, but the mood was tense. In the main shopping street, Ermou, a police team began to assess the destruction.
"It is quiet now but I've never seen anything like it in my life. People overreacted," said Yiorgos Ganatsikos, 52, a kiosk owner. "I hope they don't continue. Otherwise, God help us."
With a 24-hour general strike scheduled for Wednesday against pension reforms and the government's economic policies, many Greeks fear the demonstrations could last for days.
The shooting lit a powder keg of resentment among youth, angry at a widening gap between rich and poor. Violence at student rallies and fire bomb attacks by anarchists are common.
University professors started a three-day walkout on Monday and many school students stayed away from class in protest.
"It could have been our brother. It could have been our fellow student, it could have been one of us," said Vangelis Spiratos, 13.
Ignoring government appeals for calm, leftist demonstrators and anarchists staged running battles with police after the teenager's killing late on Saturday, which shocked the nation.
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 Athens' volatile Exarchia district.
A police official said the officer had described firing warning shots, but witnesses told TV he took aim at the boy, identified as Alexandros Andreas Grigoropoulos.
Violence spread across the country, as far as the northern city of Thessaloniki and the tourist islands of Crete and Corfu, leaving at least 34 injured. Police detained 20 in Athens.
On Sunday, protesters chanting "Cops, Pigs, Murderers" rained petrol bombs down on rows of Athens riot police, while helicopters hovered overhead and tear gas choked the city.
Scores of shops and more than a dozen banks were torched in the capital's busiest commercial districts ahead of the busy Christmas period. The mayor of Athens postponed the launch of holiday festivities.
In Thessaloniki, more than 1,000 protesters clashed with police, set fire to a bank and smashed several stores. 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. On Corfu, protesters smashed up four cars and two shops, and an 18-year-old woman was injured.