The fire, which began in the early hours, was brought under control after 18 firefighters battled the blaze for two hours with all employees evacuated from the building, the group said.
A number of people were taken to hospital with respiratory problems. A police investigation is under way to determine who was behind the incident, Al Jazeera reported.
A preliminary police account said the remnants of three gas canisters were found in the building between the basement and first floors of the office building.
Proprietor Nikos Chatzinikolaou said on Twitter he believed the company was deliberately targeted, but did not say by who.
“Explosions and fire at Real FM and Realnews! They’re burning us! They are trying to shut us down,” he wrote on Twitter.
MA/PR