A makeshift bomb exploded at the offices of the far-right political party Golden Dawn near Athens early on Tuesday, ripping through a wall and smashing the windows of a nearby building but causing no injuries, police said.
Golden Dawn has surged in popularity during Greece's debt crisis and was catapulted from obscurity to winning 7 percent of the vote in parliamentary elections in June, riding a wave of public anger at austerity, corrupt politicians and immigrants.
Activists and politicians have called for the ultra-nationalist party, whose members have been seen giving Nazi-style salutes and whose emblem resembles a swastika, to be banned.
The dynamite-packed device was placed outside the party's local offices in Aspropyrgos, an industrial suburb west of Athens.
A police official, who declined to be named, said the attack was most likely carried out by a far-left group. "It was a powerful blast that caused a lot of damage," he said. "It looks like (domestic) terrorism."
Bomb disposal experts were called to the site and Golden Dawn lawmaker Ilias Kasidiaris, the party's spokesman, rushed to the scene dressed in a white forensic suit.
Golden Dawn says it wants to rid Greece of all foreigners including what it calls the "stench" of immigrants.
Latest opinion polls show the popularity of the party - the first ultra-nationalist group to enter parliament since a military junta was overthrown in 1974 - has risen since the election.
A survey by VPRC, an independent polling company, put support for Golden Dawn at 14 percent in October, making it the third biggest party in the assembly.
The party denies it is neo-Nazi and frequently accuses journalists and critics of mudslinging and misrepresenting it.
Explosions of small homemade bombs - usually gas canisters or explosives packed together - are frequent in Greece, which is in its fifth year of a recession that has left one in four jobless and eroded living standards.