The blast occurred at 4:35 a.m. (0145 GMT) Friday in the central Ambelokipi district, and followed an anonymous warning call to two Athens newspapers, a police spokeswoman said.

Magdalini Bakalarou said the bomb "caused serious damage" in an area round the tax office, whose entrance bore the brunt of the blast. Nearby buildings, including a nearby McDonalds restaurant, and surrounding apartments had their windows blown in.

There has been no immediate claim of responsibility for the bombing. Last month, a far-left militant group called Armed Revolutionary Action said it carried out a similar attack on another Athens tax office that caused no injury.

After years of relative calm, Greek militants have stepped up attacks following the fatal police shooting of an Athens teenager in December, which sparked the country's worst rioting in decades.

In the bloodiest incident, gunmen shot dead an anti-terrorist officer guarding a witness in Athens on June 17. A group called Sect of Revolutionaries, which emerged shortly after the riots, has claimed responsibility for the shooting.

Greece was plagued by far-left militant violence in the last quarter of the 20th century, suffering a spate of killings and attacks. The country's most lethal group, November 17, was broken ahead of the Athens 2004 Olympics, and smaller ones were either eradicated or disbanded.

Also early Friday, suspected anarchist arsonists set off a small incendiary device outside an immigration policy center in central Athens, causing damage but no injury.

Small anarchist groups frequently carry out overnight arson attacks on symbols of wealth and state power.