Policemen stand outside the Pan-Hellenic Socialist Movement (PASOK) offices after a gunman opened fire at the bus, left, in Athens, on Tuesday, Jan. 10, 2017. Authorities say a police officer has been wounded after shots were fired outside the offices of the political party in an area of Athens where anarchist groups frequently clash with police and attack symbols of authority. (AP Photo/Thanassis Stavrakis)

ATHENS, Greece (AP) — A police officer was wounded in central Athens as shots were fired outside the offices of a political party early Tuesday, authorities said.

The officer was hit by shrapnel after a gunman opened fire at a police bus outside the main offices of the Pan-Hellenic Socialist Movement, a center-left opposition party, in an area of Athens where anarchist groups frequently clash with police.

"This was a murderous, terrorist attack — an attack against the police and the (party) they were guarding, an institution of our democracy," party leader Fofi Gennimata said.

The national police officers' association said the wounded policeman was being treated at a military hospital for shrapnel wounds to the chest and leg.

There was no immediate claim of responsibility for the attack in early-morning darkness. It occurred days after the arrest of a fugitive member of the Greek far-left militant group Revolutionary Struggle, which has carried out attacks including firing a rocket at the U.S. Embassy in Athens in 2007.