A Greek journalist has been shot dead outside his home in Athens, the first murder of a reporter in the country in more than 20 years.

Sokratis Giolias, 37, died after being shot by a group of unknown gunmen more than 15 times on Monday morning in the capital's eastern neighbourhood of Ilioupoli.

"Somebody wanted to silence a very good investigative reporter who had stepped on a lot of toes with his stories," Panos Sobolos, president of the Athens journalists' union, said.

Police are examining a possible link between the killing and a far-left extremist group called the Revolutionary Sect, which shot dead an anti-terror officer guarding a witness in a trial, in June last year.

'Cowardly murder'

Authorities said Giolias was shot with a pair of nine-millimetre handguns which the sect group had used in the shooting last year.

"Examination of 16 nine-millimetre Parabellum cartridges found on the scene of today's homicide ... shows they were fired by two weapons used in the activities of the Revolutionary Sect group," police said in a statement.

The sect emerged followed Greece's widespread rioting in December 2008, sparked by the police's fatal shooting of a teenage boy, and has has vowed to carry out attacks against police and the news media.

Giolias headed private Athens radio station Thema FM and wrote on a popular online news blog, Troktiko, which often deals with scandals.

According to initial reports, the gunmen asked the journalist to come out of his house, claiming someone had stolen his car, and then opened fire.

His murder was the first killing of a journalist in Greece since the mid-1980s, when left-wing urban guerrilla group November 17 assassinated a conservative newspaper publisher.

The blog said two or three gunmen were believed to have shot Giolias, who was married with a young child.

A car reported stolen was found burned out near the scene of the shooting, police said, but added they were still examing whether it was linked to the attack.

Journalist groups, the government and politicians condemned the attack.

"Democracy and freedom of speech cannot be gagged, terrorise or intimidated," government spokesman Giorgos Petalotis said.