ATHENS, Greece (AP) -- 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.

Police spokesman Panagiotis Stathis said between 15 and 20 shots were fired at the officer by at least three gunmen at about 06:20 A.M. (0320 GMT) in the residential district of Patisia.
“It was a cold-blooded murderous attack,” said Stathis, adding that the attack was not preceded by any warning call and that there had been no immediate claim of responsibility.
Greek domestic terrorist groups have stepped up attacks since the country was struck by massive riots in December, triggered by the fatal police shooting of a teenage boy. But Wednesday's attack marked a severe escalation, and came a few days before the long-anticipated opening of the New Acropolis Museum, which is expected to be attended by dozens of dignitaries from across the world.
The officer had just taken over the morning shift of guard duty outside the home of a 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 attempt apparently made to approach the home of the witness.
Coroner Philippos Koutsaftis told local media that 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,” Koutsaftis 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.
The officer, a 41-year-old father of one, 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.
Forensic experts were collecting evidence from the scene, and the officer's body was not removed for more than two hours.
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 as he stood guard outside the Culture Ministry 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 a newly appearing group calling itself Sect of Revolutionaries have made threats against police and have warned they would escalate attacks.