ATHENS, Nov. 6 -- Greek opposition socialist PASOK party's headquarters came under an armed attack on Monday, with no injuries reported, police said.

According to early information, at least two perpetrators opened fire with semi-automatic rifles from a car and fled the scene.

The target according to police sources was the police bus which was parked outside the building. Eyewitnesses said they heard at least four shots.

According to party spokesman Pavlos Christides, there were about 40 people inside the building during the attack.

"Unfortunately we have witnessed again attacks. Every week we face incidents. An end should be put. It is a miracle we haven't mourned victims," he told local TV station ANT1 shortly before the start of a debate between candidates for the leadership of an under- construction coalition of the centre-Left.

In an e-mailed statement Greek Prime Minister Alexis Tsipras condemned the attack.

During a similar attack against PASOK's premises last January a policeman was slightly injured.

The headquarters of the socialist party, which ruled Greece for several years since 1981, have been targeted numerous times in recent years by anarchists, who have been operating in the nearby Exarchia area for decades, most often with petrol bombs.

Opposition parties have called on the government to take more measures to address the anti-establishment groups-linked security issues in the center of the Greek capital.