Greece's anti-terrorist police launched a major manhunt on Wednesday after an officer guarding a witness whose testimony led to the jailing of left-wing extremists was gunned down on duty.

Police said they believed a far left group, Revolutionary Sect, which emerged in February, killed Nektarios Savas, 41, a married man with a child.

Savas was riddled with bullets by three assailants on motorbikes who opened fire on him as he sat in his car in the northern Athens district of Ano Patassia.

Police have become a particular target of extremist groups since last December when violent riots broke out across Greece in protest over the killing of an Athens teenager by a police bullet.

Savas, who was in plain clothes and an unmarked vehicle, had just come on duty outside the home of Sofia Kyriakidou, a state witness in the 2004 trial of four followers of People's Revolutionary Struggle (ELA).

The four, including Kyriakidou's ex-husband, were sentenced to 25 years in jail but later released, mostly for health reasons.

ELA, Greece's most notorious extremist outfit after the dreaded November 17 group, ceased its activities in 1995 after some 250 attacks on police, banks, government offices and US interests.

But other radical groups such as Revolutionary Sect have emerged since then, while Revolutionary Struggle (EA), which has been active since 2003, stepped up its attacks, seriously wounding a police officer on duty outside a government ministry in January.

Police spokesman Panagiotis Stathis told a press conference Wednesday that 24 cartridge cases were found at the scene after the shooting of Savas, which were identified as coming from a weapon already used by Revolutionary Sect.

Revolutionary Sect burst on to the scene with the machine-gunning of a police station and the headquarters of a private television channel in the Athens area at the beginning of February.

Its claim of responsibility warned of further indiscriminate attacks on police.

President Carolos Papoulias and government spokesman Evangelos Antonaros strongly condemned Wednesday's attack, which Stathis said was the third on police since December.

Antonaros hastened to add that there was no link with the high-profile opening of new Acropolis Museum on Saturday and no threat to the guests attending the inauguration.

The attacks have forced police back to the drawing board less than a decade after the 2002 dismantling of Greece's deadliest far-left organisation November 17, blamed for 23 murders between 1975 and 2000, including two police officers.

"Setting fire to a car is one thing, but here we are dealing with something more worrying -- pre-planned operations by organised groups," police spokesman Stathis said earlier this year.

A prominent criminologist, Yiannis Panoussis, has said that these extremist groups "feel legitimised by the December crisis" after the teen's murder and have "crossed a line into more stringent and blind violence."