Unknown gunmen shot and seriously wounded a policeman in Athens on Monday, the second such attack since police shot dead a teenager last month prompting Greece's worst riots in decades.

At least two assailants repeatedly fired semi-automatic weapons at a group of riot police guarding the Culture Ministry in the central Athens Exarchia district where the 15-year-old was killed on Dec. 6.

No one has claimed responsibility for the shooting, but police said it bore the hallmarks of a Dec. 23 attack on a police bus which a new leftist group said it staged to protest the teenager's killing.

"The people that carried out this attack targeted democracy," Interior Minister Prokopis Pavlopoulos told reporters. "No bullet or murderer can hurt the police's morale."

About 40 shots were fired from at least two weapons, including an AK-47 assault rifle and a 9 millimetre gun, police said. The gunmen also tossed a hand grenade at police to cover their escape.

A 21-year-old policeman was taken to hospital with serious injuries from three bullets.

"His condition is critical but stable ... This dramatic incident was a strike against democracy, against society and its institutions," Health Minister Dimitris Avramopoulos told reporters outside the hospital.

Greek police chief Vassilis Tsiatouras said shells found at the site of the pre-dawn shooting matched the weapon used in the Dec. 23 attack. Leftist group, Public Action, said it was responsible for that attack in which no one was injured.

Police said they took in 72 people for questioning, but no arrests have been made. The gunmen fled into the streets of Exarchia where the violence erupted last month. Police cordoned off the area and helicopters hovered overhead.

Athens was hit with weeks of riots after the killing of Alexandros Grigoropoulos unleashed lingering anger at high youth unemployment and economic measures.

The violence has shaken a conservative government that has a fragile one-seat majority and trails the opposition in opinion polls by around six percentage points. Some analysts say months of street protests could force early elections.

Political parties condemned Monday's attack, calling for a swift investigation and for the culprits to face justice.