ATHENS (Reuters) - Shots fired at riot police guarding the headquarters of Greece’s Socialist PASOK party on Monday came from the same gun that one particular guerrilla group has used in attacks, police said.

Police investigators search for evidence following a gun attack outside the headquarters of Greece's socialist Pasok party in central Athens, Greece, November 6, 2017. REUTERS/Costas Baltas

There were no injuries during the shooting, which occurred at a busy intersection in Athens at around 1745 GMT, when some 30 people were inside the party’s offices. No one has claimed responsibility.

Police found four shells at the spot and said tests showed they had been fired from a Kalashnikov AK-47 rifle. One bullet passed through a shop window and a second pierced a metal electricity pole.

The same rifle had been used in at least two attacks claimed by left-wing guerrilla group Revolutionary Self-Defense, including one on the PASOK offices in 2014, police said.

The group, which emerged during Greece’s economic crisis, claimed an attack with a hand grenade on the French embassy in Athens last year.

“The tests point at a certain direction and the government needs to act,” PASOK spokesman Pavlos Christides said.

Small-scale attacks on businesses, police, politicians and embassies are relatively frequent in Greece. The country has a long history of political violence and is struggling to emerge from a huge debt crisis that has sent unemployment soaring and cut incomes.

PASOK, a once dominant party that signed up to the country’s first international bailout in exchange for austerity, holds a leadership contest on Sunday. It has seen its popularity wane since 2009 when it last won a national election with 43 percent.

Its headquarters are in the Athens neighborhood of Exarchia, the scene of regular confrontations between police and anti-establishment protesters.

(This version of the story was corrected to make clear that same rifle was used in past attacks)