Athens - Days after the worst riots in decades, Greek Prime Minister Costas Karamanlis said on Tuesday he underestimated the magnitude of the problems plaguing his country just as students firebombed Athens' police headquarters.

"I underestimated the scandals of the past few years. This was my mistake," Karamanlis said, addressing his ruling conservative party's parliamentary group.

While Karamanlis admitted negligence and wrongdoing on behalf of his government and promised more reforms, masked youths continued their attack on riot police for a second straight week, attacking capital's police headquarters.

On Tuesday, police say a group of 30 youths threw petrol bombs and stones at the central Athens building, causing damage to the building and to police vehicles parked outside.

In other parts of Athens, students blocked streets outside the city's main court house and maximum security prison to protest against the police shooting of a 15-year-old boy on December 6 which sparked a week of riots.

While the shooting sparked the violence, students have gone on to protest the fate of young Greeks reeling under economic hardship and the impact of a global recession on Greece's 240-billion-euro (330- billion-dollar) economy.

Widespread anger has grown over the economic polices of the conservative government, which have caused a widening social gap and rising unemployment.

The official unemployment figure is near 9 per cent and rising, leaving many young people disillusioned. Students have expressed anger over investing a lot in their education, but with few prospects at the end of it.

The minimum wage is 700 euros a month, and nearly one-fifth of the population lives below the poverty line. Many Greeks work a second or third job to make ends meet.

Damage from the days of rioting in Athens alone has been estimated at more than 200 million euros. More than 500 people have been arrested.

While the intensity of the protests has largely died down, students, unions and leftist groups have called for more demonstrations on Wednesday and Thursday against education and pension reforms, privatizations and rising taxes.

In the unrest, more than 600 businesses have been destroyed nationwide, resulting in more than 200 million euros in damage.

Shop owners said business has dropped 90 per cent in the centre of Athens, while tourism is down by 30 per cent.

An opinion poll published Sunday by Kathimerini newspaper said public disapproval at the government had reached 68 per cent, with 60 per cent of those polled saying the riots were a social uprising rather than an outburst by an isolated fringe of violent protesters.

Despite widespread criticism of the prime minister's handling of the riots, he dismissed the idea of early elections.

The two policemen charged with killing the teenager have been jailed pending trial. One claimed he fired warning shots after being attacked by youths in Exarchia, a neighbourhood frequented by leftists and self-styled anarchists. (dpa)