About 30 people were involved in the attack at 5.15am local time, a police official said. There were no injuries.
In a statement published on the Internet, the Rouvikonas group said the attack was linked to a probe into allegations of bribery by the drug-maker to doctors and public officials.
The Greek parliament voted on Thursday to probe the role of 10 politicians in the case. They have all denied any wrongdoing, saying the case is an attempt by authorities to discredit opposition politicians a little over a year before scheduled elections. Prosecutors are already investigating the role of non-politicians.
Novartis said that it condemns any act of violence and urged Greek authorities to guarantee its employees’ safety.
“We are particularly worried following the attack,” Novartis Hellas said in a statement.
“Novartis vehemently condemns any act of violence and blackmail or any destructive act. We call on Greek authorities to condemn the incident and safeguard our people’s safety, their protection is our priority.”
The drug-maker has promised to take “fast and decisive action” should the investigation in Greece find that its managers engaged in unethical or illegal conduct.
Small-scale attacks on businesses, state buildings, police, and politicians are frequent in Greece, which is emerging from its worst debt crisis in decades and has slashed healthcare spending, wages, and pensions to shore up its finances.