Protesters interrupted footage of a speech of Mr Karamanlis
Protesters in Greece have interrupted a TV news bulletin to call for the continuation of mass protests over the recent killing of a teenager by police.
Ten youths briefly seized a state NET TV studio, holding banners saying "Stop watching, get out onto the streets".
Separately, youths attacked the riot police headquarters in the capital, Athens, damaging several vehicles.
Some 70 people have been injured during riots across Greece sparked by the shooting of the teenager on 6 December.
It is estimated that 400 people have been detained during the protests.
Youths also clashed with police in the northern city of Thessaloniki
The policeman accused of shooting Alexandros Grigoropoulos, aged 15, has been charged with murder.
The shooting has also generated widespread anti-government sentiment, with many protesters voicing their anger over the government's economic policies.
Conservative Prime Minister Kostas Karamanlis has rejected calls to step down.
He says the country needs a "steady hand" to deal with the economic downturn, "not scenarios about elections and successions".