Members of the Rouvikonas (Rubicon) group broke the entry box and windows of the offices with a sledgehammer and threw red paint before they fled.
Greece's parliament on Thursday launched a probe into claims that nearly a dozen senior politicians received bribes from or helped promote Novartis during their term in office.
Among the 10 senior politicians reportedly named by protected witnesses in a mostly US-based investigation are conservative ex-PM Antonis Samaras, EU migration commissioner Dimitris Avramopoulos and central banker Yannis Stournaras.
They all deny wrong-doing.
The Swiss drug-maker is suspected of having bribed decision-makers and doctors between 2006 and 2015 to build a commanding position in the Greek health market, as well as secure inflated prices for its products.
Rouvikonas claimed the attack in a post on an anti-establishment website saying it came in support of those implicated because they face allegations from anonymous witnesses.
Novartis issued a statement Sunday asking Greek authorities to denounce the Rouvikonas attack and protect the lives of its employees.