Violence continued for a fourth day in Athens, as a funeral was held for a teenager whose death has sparked rioting across Greece.
Clashes erupted near the cemetery where 15-year-old Alexandros Grigoropoulos, shot by police on Saturday, was buried.
Youths also fought police outside parliament, in a repeat of the violence that has seen hundreds of buildings torched and dozens injured.
The opposition said the government had lost public support and should resign.
On Wednesday union leaders plan to hold a 24-hour general strike over welfare reforms. Police fear the stoppage, which is expected to bring the country to a standstill, could fuel further violence.
Funeral clashes
Fresh protests began in central Athens early on Tuesday. Schools were shut as thousands of teachers, schoolchildren and parents marched on parliament to protest against the killing.
The situation escalated as hundreds of young people joined the protest, throwing stones and bottles at lines of riot police, who responded with tear gas.
Police used tear gas to disperse stone-throwing protesters
In the afternoon thousands of mourners gathered for the teenager's funeral in a coastal suburb further south.
The ceremony was calm, but violence then erupted outside the cemetery. Police used tear gas against groups of youths who threw stones and set rubbish bins ablaze.
By late evening, 15,000 police were deployed in the capital to maintain control, Reuters news agency said.
There was more fighting elsewhere in the country too.
In Thessaloniki, police clashed with groups of young people following a protest march earlier in the day. In Patras, a western port, rioters armed with petrol bombs and stones attacked the main police station.
Two police officers have been charged in connection with Alexandros Grigoropoulos's death, but results of a post-mortem to determine the trajectory of the bullet that killed him are not yet known.
The officer who fired says it was a ricochet from a warning shot but witnesses told Greek TV he fired directly at the teenager.
Lost confidence
In Athens, Prime Minister Costas Karamanlis held talks with President Karolos Papoulias and opposition leaders to discuss what action to take.
Mr Karamanlis, whose conservative party has a parliamentary majority of just one seat, called for unity and said rioters would not be shown any leniency.
"No one has the right to use this tragic incident as an excuse for acts of violence," he said.
But socialist leader George Papandreou said Greeks had lost confidence in the government.
"The only thing this government can offer is to resign and turn to the people for its verdict," he said.
'Rage'
Scores of arrests have been made since Saturday. Protesters wielding petrol bombs have set fire to banks, shops, hotels, vehicles and even the giant Christmas tree in Athens' central Syntagma Square.
Violent clashes have been reported in towns and cities across the country.
Appeals for calm have so far been largely ignored. Police appear to be powerless to prevent rioters from attacking symbols of wealth and prestige in Athens, the BBC's Malcolm Brabant reports.
"Rage is what I feel for what has happened, rage," said one protesting student. "This cop who did it must see what it is to kill a kid and to destroy a life."
Mr Karamanlis has blamed "extreme elements" for taking advantage of the situation to engage in vandalism and pledged to compensate damaged businesses.
Observers say a state of emergency may be imposed, giving the authorities special powers to clear the streets.
But there is no question of calling in troops, our correspondent says: Greece has bitter memories of military rule so seeing troops on streets would be beyond the pale.
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