"A makeshift time-bomb exploded outside the office of Golden Dawn, there are no injuries," a police official said. Police said there was no claim of responsibility for the blast on the fifth floor of the building near the central Athens Omonia Square.

There has been a series of gas canister and bomb attacks by leftist and anarchist groups in Greece since the fatal police shooting of a teenager sparked the country's worst riots in decades in December 2008. Police said there was a warning call to a newspaper about a suspect box placed outside the office of ultra nationalist group Golden Dawn.

"All these actions serve as negative arguments for our country," government spokesman George Petalotis told Greek TV Sky.