Armed attackers drove a van loaded with incendiary devices through the main entrance to Microsoft's main offices in Athens early Wednesday, setting off a blaze that burned through the building's lobby area.

No injuries were reported in the pre-dawn raid on the office, home to about 100 Microsoft employees, and no group has come forth to claim responsibility. Greek authorities said no prior warning of the attack was given.

"Our priority is our people, and we are grateful that there were no injuries and all the employees are safe," said a local Microsoft spokesperson. Reports said the ground floor of the building, located in Athens' Maroussi suburb, was heavily damaged and would be shuttered for at least the rest of Wednesday.

Police said three gunmen ordered security guards to stand aside before ramming the van, loaded with gas canisters, into the building's entryway and setting it ablaze. Total damage was initially estimated at about $75,000.

[ Learn about how technology can help prepare for ground attacks. See DHS Simulation Software Predicts Bomb Blast Paths. ]

There was no apparent motive for the attack, but economic unrest in Greece, which recently negotiated a $162 billion bailout package with the European Union and International Monetary Fund in exchange for austerity measures, has stirred unrest against corporations, including high-profile American companies.

Facilities owned by McDonald's, Citi, and Starbuck's have all been hit by arson attacks in recent months. Restaurant workers in Greece staged a 24-hour walkout Wednesday to protest wage reductions and other cutbacks that they blame on the bailout.

Despite its struggling economy, Greece remains an important market for Microsoft. Major customers there include Chandris Hotels and Resorts, Hellenic Petroleum Company, the Athenaeum InterContinental Hotel, and the National Bank of Greece. The bank's chairman and Greek finance minister designate, Vassilis Rapanos, resigned Wednesday, citing ill health.

A recent security study by Microsoft found that the frequency of malware attacks on computers in Greece is above the global average. The company's data showed that in the fourth quarter of 2011 an average of 8.5 out of every 1,000 PCs in the country was infected, compared to a worldwide average of 7 per 1,000.

Black Hat USA Las Vegas, the premiere conference on information security, features four days of deep technical training followed by two days of presentations from speakers discussing their latest research around a broad range of security topics. At Caesars Palace in Las Vegas, July 21-26. Register today.