Greek counter-terrorism experts are hoping that a man who survived a drive-by shooting at a branch of the far-right Golden Dawn party in Athens will help shed light on the assailants' identity.

As supporters of the extremist group held a memorial event on the spot where two of its members were shot dead on Friday, there was growing speculation that far-left radicals, seeking revenge for the murder of an anti-fascist musician, were behind the attack.

The man who survived, by diving into a building, is believed to have come face to face with the assassins.

"He is our best hope," said one source. "He's the one who got a close-up view of them."

The brazen nature of the attack has sparked fears that with the nation mired in economic crisis and political divisions growing, Greece is being pushed into a cycle of violence.

The two victims, identified as Manolis Kapelonis, 23, and Giorgos Fountoulis, 26, and described by Golden Dawn as "members of our family", were shot dead after two men on a motorcycle approached one of the party's local offices and fired indiscriminately at the building.

A third man, Alexandros Gerontas, was severely injured in the assault. After extensive surgery, doctors said his condition was improving.

Gerontas's mother, who has been praised for her dignified response and outspoken rejection of the course the country is taking, has issued a televised appeal for unity.

"I want to send a message especially to our youth, who are going through such difficult times, not to create such extremism," she said. "Bloodshed is not the right way. Where do we live? In a jungle?"

Outrage has been voiced by politicians across the spectrum amid concerns that the attack, which lasted 10 seconds and was captured on CCTV, will intensify what has been likened to a civil war between radical factions on the left and right.

Parties that only weeks ago were deploring the neo-fascist Golden Dawn have called it an "assault on democracy".

The shooting comes barely two months after Pavlos Fyssas, a leftwing hip-hop artist, was stabbed to death by a self-confessed member of Golden Dawn in another Athens suburb.

The murder elicited nationwide protests and an unprecedented crackdown on the extremist group that has since seen its leader and two other senior members imprisoned on charges of operating a criminal gang.

"The undeclared, low-grade civil war that we are living should not be declared officially," wrote Christos Dervenis, a leftwing commentator, in the Ethnos newspaper. "Our democracy may be strong but it is not a given."

Others warned that Golden Dawn, which has seen its support drop dramatically since the clampdown – even if it remains Greece's third biggest political force – would use the attack to gain sympathy in the polls.

"They were executed. If Greeks saw the images, they would be shocked," said the party's press spokesman, Ilias Kasidiaris.

Ballistics experts said that the professionalism of the assailants – the victims were killed at point blank range when one of the gunmen got off the bike and approached the building – left no doubt that guerrillas were behind the attack. Twelve bullets fired from a Serbian-made 9mm Zastava handgun were found at the scene.

"Whoever is behind this attack had a message and it has been heard," said Mary Bossis, professor of international security at the University of Piraeus.

"In the absence of any group assuming responsibility all options are open," she added. "In Greece today there are many groups that are politically violent and very active."