Four assailants on two motorbikes opened fire on the police station in the northern suburb of Agia Paraskevi on Tuesday night. A total of 99 cartridges were later recovered from the scene, with a police source saying two Kalashnikov rifles were used in the attack.
Investigators also found a grenade pin, but it was not immediately clear if one of the officers had used a flash grenade against the attackers.
A trainee policewoman, who sustained damage to the lung after being shot in the back, was in serious condition, needing an all-night operation to save her life, police said.
Another patrolman was shot in the abdomen and legs, while the remaining four sustained lighter injuries. A passerby was also hospitalised after suffering a nervous collapse following the attack.
Police said the strike had been carefully timed to coincide with a guard change outside the station and caught the officers in the open.
It also came on the eve of a national holiday commemorating Greek resistance to Fascist Italy in World War II and a day after the government put a bounty on three bank robbery suspects believed to have links with far-left extremism.
President Carolos Papoulias condemned what he called "the cowardly terrorist attack" and expressed sympathy for the victims.
Prime Minister George Papandreou visited the injured officers in hospital on Wednesday before departing for a scheduled visit to Sweden.
Minister for the Protection of Citizens Michalis Chryssohoidis, who also visited the wounded with Health Minister Mariliza Xenoyannokopoulou, called the attack "one hundred percent terrorist."
Promising that Greek society "will not let itself be terrorised," he appealed to the people to help the police arrest those responsible.
There was no claim of responsibility, although similar attacks have been carried out in the past by two militant groups.
They are the extreme-left group Revolutionary Struggle, active since 2003 and listed by the European Union and United States as a terrorist organisation, and the Revolutionary Sect which was set up earlier this year.
Revolutionary Sect has threatened to indiscriminately kill police officers, with a letter sent in February warning that "from now on, the life of every cop is not worth more than a bullet."
Left-wing extremists have launched a number of attacks on police targets since last December's riots which were triggered by the fatal shooting of a 15-year-old youth.
But Monday's attack was the most damaging assault on a police station.
Revolutionary Struggle in January ambushed a police patrol and nearly killed a young officer. In June, the group murdered an anti-terror officer outside the home of a witness in the trial of another left-wing militant group.
The socialists came to power earlier this month with pledges to crack down on left-wing militancy.
Chrysohoidis was public order minister in 2002 when police succeeded in dismantling the country's deadliest far-left organisation -- November 17 -- blamed for 23 murders between 1975 and 2000, including two police officers.
But the recent attacks have put renewed pressure on the police and Chrysohoidis told parliament last week that the department "has gone to pieces."
Police were heavily criticised in December for failing to stop the riots and looting that followed the teenager's fatal shooting.
The department is nominally headless since last week when Chrysohoidis sacked its chief over the arrest of a number of left-wing figures which the minister criticised as 'over-zealous'.
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