All flights in and out of Greece have been cancelled as a result of the widespread unrest in the country as Prime Minister Costas Karamanlis tries to reassert his authority.

Mr Karamanlis held crisis talks overnight with his cabinet and emerged after three hours promising to punish those who incite violence.

The meeting came amid fresh unrest in Athens as thousands of people demonstrated outside parliament in a protest timed to coincide with the funeral of 15-year-old Alexandros Grigoropoulos, who was shot by police over the weekend.

Demonstrators hurled petrol bombs and other missiles in a bid to breach a cordon around parliament and other official buildings in Athens and Salonika.

Meanwhile hundreds of people, including many students, attended the funeral in the Athens seaside suburb of Palio Faliro.

Some youths shouted anti-police slogans at the cemetery but most people respected the family's request for a solemn service.

Mr Karamanlis said the rioters who had staged protests for the past four days were "enemies of democracy" and vowed to end the unrest.

He made a fresh appeal for national unity after the chaos that has engulfed the country following the fatal police shooting on Saturday.

"All those who cause violence and vandalism are enemies of democracy," he said. "National unity is a common debt.

"The struggles of workers and the death of this boy cannot be confused with acts of vandalism."

But the main opposition party says the Government has lost control of the country and should resign.

After three days of disorder across the country, Greece's socialist opposition leader George Papandreou accused the Government of failing to protect the country as it entered the fourth day of the worst civil unrest in more than three decades.

He called on Mr Karamanlis to resign and hold an immediate election to allow democracy to take its course.

Police reported their headquarters in the Greek port of Patras was the latest to be under siege by around 500 demonstrators who were attacking with stones and petrol bombs, with riot police firing tear gas and trying to hold them back.

Another student demonstration was scheduled for Wednesday (local time) ahead of a general 24-hour strike over the Government's economic reforms.

-ABC/AFP