Gunmen shot dead an anti-terrorist police officer guarding a witness in central Athens early Wednesday, in an escalation of domestic terrorist attacks in the country.

Greece's conservative government denounced the "cowardly attack." There was no claim of responsibility but police matched bullets from at least one of the weapons used to previous attacks carried out by Greek far-left militant group Sect of Revolutionaries.

Police spokesman Panagiotis Stathis said between 15 and 20 shots were fired at the officer by at least three gunmen at about 6:20 a.m. in the residential district of Patisia.

"There was no warning, no telephone calls," Stathis said. "This was a cold-blooded murder."

It was the first targeted killing attributed to domestic terrorism in years.

Greek domestic terrorist groups have stepped up attacks, particularly against police targets, since massive riots in December triggered by the fatal police shooting of a teenage boy. But most have been late-night bombings that have caused no injuries.

"From one minute to the next, after the events of December, the police force was suddenly on the ropes, in a situation where all of its actions were viewed in a negative context," Stathis said.

"These people will be caught and brought to justice."

The slain officer was identified as 41-year-old Nektarios Savvas, a father-of-one.

The officer had just taken over the morning shift of guard duty outside the home of a key witness in the trial of the far-left Greek terrorist group Revolutionary Popular Struggle, known by its Greek acronym ELA. Only the officer was targeted in the attack, with no apparent attempt to approach the witness' home.

Coroner Philippos Koutsaftis said the officer had died of multiple gunshot wounds to the body and head.

"He was carrying a gun which was holstered. He was hit many times by shots that appear to have been fired at close range," he said.

Before disbanding in 1995, ELA was blamed for killing a police officer and a Supreme Court prosecutor, as well as scores of bombings over 20 years. More than 30 attacks were aimed at American targets, mostly in the early 1980s, including the bombing of the U.S. ambassador's residence, embassy vehicles, and branches of American banks and companies.

In 2004, four people were convicted of being involved with ELA and were sentenced to 25 years in prison. But all have appealed their convictions. The witness the officer was guarding had been involved in the original trial and also testified in the appeals case earlier this year. The appeals case is expected to continue until later this year.

The officer died at the scene, police said. Television footage showed him slumped over in the driver's seat of the dark-colored unmarked vehicle, shortly after authorities cordoned off the area.

Greece has faced attacks by domestic terrorist groups for decades. But authorities believed the problem had diminished after the arrest of several members of the country's deadliest group, November 17, following a botched bombing in 2002.

However, the fatal police shooting of the teenager in December and the riots that ensued have sparked a resurgence of domestic terrorism, with attacks becoming increasingly brazen.

In January, a far-left group called Revolutionary Struggle claimed responsibility for a Jan. 5 shooting that seriously wounded a 21-year-old riot policeman in central Athens. The following month, police destroyed a car bomb containing enough explosives to crumble a four-story building which had been planted outside Citibank offices on the outskirts of Athens.

In proclamations and claims of responsibility, both Revolutionary Struggle and the newly appeared Sect of Revolutionaries have made threats against police and have warned they would escalate attacks.