The attack occurred after Greece's new Socialist government promised to crack down on anarchist and far-left militant groups that have stepped up bombings and shootings in the wake of riots in Athens and other cities in December.
Two officers were seriously wounded in Tuesday's shooting, which occurred at 9:40 p.m. (1840 GMT, 2:40 p.m. EDT) in the Agia Paraskevi area, north of the Greek capital, police said. Four other officers were hospitalized with less serious injuries, while a bystander was treated for shock, state hospital officials said.
"This was a cowardly and murderous attack ... it was 100 percent terrorism," said Michalis Chrisochoidis, a government minister in charge of police.
Two men on a motorcycle stopped and opened fire on police officers standing outside the station before driving away. The gunmen fired about 50 shots, authorities said.
Officers from the police's anti-terrorism division gathered evidence at the targeted police station, while police stopped and searched vehicles across northern Athens.
No group claimed responsibility for the shooting, but suspicion fell on far-left militant groups that have increased attacks since the December riots that were triggered by the fatal shooting of a teenage boy by police.
In June, gunmen shot dead an anti-terrorist policeman guarding a witness in central Athens. A group called Sect of Revolutionaries later claimed responsibility, saying it acted to avenge the teenage boy's death.
The country's Socialist government, which won general elections earlier this month, has promised to crack down on domestic terrorist groups.
Police were also bracing for potential violence on the Dec. 6 anniversary of the police shooting that sparked the riots in Athens and other Greece cities.
"These cowardly killers will be caught and brought to justice," Chrisochoidis said.