The attackers, suspected of being members of the far-right, stormed the Favela Free Social Centre in the port city near Athens, holding torches and iron bars.
Two people were taken to hospital.
"Today in the afternoon while we opened our centre for our weekly meeting, we were attacked by a group of Golden Dawn members with iron bars, helmets and flares," a post on the Facebook page of the Favela Free Social Centre read, referring to Greece's far-right, neo-Nazi party.
Among those wounded by the attackers was a woman that had a connection with Pavlos Fyssas, the anti-fascist rapper killed by Golden Dawn members in September 2013, according to police sources.
A total of 70 Golden Dawn lawmakers and party officials are on trial on a wide range of charges, including membership of a criminal organisation linked to the fatal stabbing of Fyssas and several other charges.
The neo-Nazi party remains the fourth largest in the Greek parliament.
The Favela centre was also attacked last August, and four people were arrested by the counter-terrorism unit, according to Athens News Agency.