Greek police say they face growing public hostility after the recent riots
A Greek riot policeman has been left seriously injured after being shot in central Athens.
Greece's police chief says bullet cases matched the Kalashnikov rifle used in another attack on police in December.
The officer, 21-year-old Diamandis Matzounis, was part of a unit guarding the culture ministry when the pre-dawn attack took place.
Last month Greece saw its worst riots in decades after the fatal shooting by police of a teenager in Athens.
A huge manhunt is under way, following the latest attack.
A number of people have been taken in for questioning.
The victim was on a life support machine on Monday after spending five hours in surgery. He lost several litres of blood after being hit in the body and the leg, and is described as being in a critical condition.
The officer had apparently spotted the gunmen and warned his colleagues shortly before he was hit.
Greece's police chief, Lt Gen Vassilis Tsiatouras, said an automatic rifle and a handgun were used, and that a hand grenade was also thrown during the attack.
He said the rifle was the same as that used on 23 December, when two gunmen hidden within the grounds of Athens University opened fire at a riot police bus as it passed by.
None of about 20 police on board the bus at the time were injured.
A group calling itself Popular Action claimed to be behind that attack.
But General Tsiatouras played down suggestions that the shootings were carried out by a terrorist organisation, saying he thought they were the work of a group of people who had "lost their senses" following the police killing of 15-year-old Alexis Grigoropoulos.
The incident took place in the Exarchia district, close to where Grigoropoulos was shot last month.
His death sparked weeks of clashes between rioters and security forces.
The policeman accused of shooting the teenager has been charged with murder.
The BBC's Malcolm Brabant in Athens says police have been subjected to almost daily attacks from rioters since Grigoropoulos was killed.
Our correspondent says there is a fear that the current vehement anti-establishment climate in Greece may lead to a resurgence in domestic terrorism.