Unidentified attackers have opened fire on the headquarters of Greece's governing New Democracy party with a Kalashnikov assault rifle, in what the government said was a worrying escalation in political violence.

Police said a bullet pierced the window of the political office that conservative Prime Minister Antonis Samaras maintains in the building, but no-one was hurt.

The assault follows a spate of bomb attacks against journalists and political figures in the past week, some claimed by leftist groups angry at Greece's financial crisis.

Greece is in the sixth year of a recession that has fuelled anger against foreign lenders and the political class, blamed by Greeks for bringing the country close to bankruptcy.

Government spokesman Simos Kedikoglou condemned the shooting, saying even a symbolic attack on the prime minister was unheard of.

"This is a new, worrying escalation of the effort to create terror in our society," he said.

Political violence is not uncommon in Greece but deadly attacks are rare.

Officials said Mr Samaras no longer uses his party office on Syngrou Avenue, which is near the centre of Athens, and he was not present at the time of the shooting.

"At about 3am (1am Irish time), guards saw two men coming out of a black car and firing with a Kalashnikov at the building, which was empty at the time," said a police official.

He said at least nine bullet casings were recovered from the scene and police were examining a burnt-out car found a few kilometres away.

Anti-terrorism police cordoned off the area and were checking security cameras near the party building.

A spate of recent attacks have targeted public figures.

Yesterday, the Athens home of Mr Kedikoglou's brother was hit by a petrol bomb and three New Democracy offices in Athens were targeted on Friday. No injuries were reported in the attacks.