A van has been driven through the front doors of the Microsoft office in Athens and an incendiary device set off that burned the building entrance, police have said.
There were no reports of injuries in the pre-dawn attack on the US company's headquarters in the Greek capital, located in the Maroussi suburb north of the city centre.
There was no immediate claim of responsibility. Authorities said no warning call had been made before the attack.
Greece has experienced attacks by several small armed anarchist or domestic terrorist groups for decades, which usually target official buildings, banks or symbols of state power with small bombs or incendiary devices. The attacks usually occur late at night and rarely cause injuries.
Police said initial information indicated three people had been inside the van. They forced the two security guards at the building to leave before they reversed the van into the front entrance, smashing the door.
The attackers then triggered an incendiary device inside the van that police said appeared to have consisted of camping gas canisters and several containers of petrol.
Although they have died down over the past year, attacks had spiked following the fatal police shooting of a teenager in Athens in December 2008 that led to widespread riots across the country.
One group that had appeared in the aftermath of the rioting had set itself apart by claiming responsibility for shooting dead an anti-terrorism police officer in central Athens in 2009 and a journalist in 2010.