Protesters forced their way into Greece's state NET television news studio Tuesday and interrupted a news broadcast featuring the prime minister so they could urge viewers to join mass anti-government demonstrations.

For more than a minute, about 10 youths blocked a broadcast showing a speech by Prime Minister Costas Karamanlis. Instead, they displayed banners reading: "Stop watching, get out onto the streets," and "Free everyone who has been arrested." No one was hurt, and no arrests were reported.

NET chairman Christos Panagopoulos claimed the protesters violently forced their way into the studio. "This goes beyond any limit," he said.

It was the latest twist in 11 days of riots and protests after a policeman shot and killed a 15-year-old boy on Dec. 6. The violent protests have evolved from being just aimed at Greek police to being highly critical of Karamanlis' conservative government.

Click here for photos.

Karamanlis has rejected mounting demands to resign and call new elections.

Earlier Tuesday, masked youths attacked riot police headquarters in Athens and protesters clashed with police in the northern city of Thessaloniki.

Police said 30 youths threw petrol bombs and stones at the riot police building, causing extensive damage to seven cars and a police bus parked outside.

Students blocked streets in Athens and dozens of teenagers gathered outside the capital's main court complex and a maximum security prison — where some threw stones at police.

Protesters have called for riot officers to be pulled off the streets, for police to be disarmed and for the government to revise its economic, social and education policies.

The protests have brought higher education in Greece to a standstill. Lessons have stopped at more than 100 secondary schools that are under occupation by students, according to the Education Ministry. Scores of university buildings across Greece are also occupied.

After the shooting death of 15-year-old Alexandros Grigoropoulos, furious youths smashed and burnt hundreds of shops in Athens' main shopping area, and attacked riot police who responded with massive tear gas.

Dozens of people have been injured in the rioting, while more than 300 people have been arrested. The policeman accused of killing the teenager has been charged with murder and is being held pending trial.

In the northern port of Thessaloniki, riot police fired tear gas Tuesday to disperse 300 youths throwing fruit and stones outside the city's main court complex. The disturbance followed a court decision that found eight police officers guilty of abusing a student following riots two years ago.

The policemen received suspended sentences ranging from three years and three months for grievous bodily harm to 15 months for being an accessory to the abuse.

In a symbolic gesture meant to revive riot-shocked Athens, city authorities will light a large Christmas tree Tuesday on central Syntagma Square — which has been at the center of many of the protests. The tree replaces one burnt during last week's riots.

Overnight, arsonists attacked three Athens banks with petrol bombs, causing extensive damage. There were no injuries or arrests. Every year, small anarchist groups carry out dozens of firebombings in Greece against government property, banks and diplomatic vehicles.