Police are investigating an arson attack on a gym and martial arts facility in Melbourne's inner west overnight.Emergency services were called to the Genesis Health Club in Hampstead Road, Maidstone, after reports of a large fire about 12:30am.Fire Rescue Victoria said crews arrived at the scene within seven minutes to find the three-storey concrete building well alight.The building contained a gym and jiu-jitsu facility.It took about an hour for fire crews to bring the blaze under control and firefighters remained at the scene this morning to put out any hot spots.Nobody was inside the building, which sustained significant damage.Victoria Police said the offenders fled the scene and investigators were treating the fire as a targeted incident.Boris, who trains at the Brazilian jiu-jitsu facility, said the martial arts centre was badly damaged by the fire."It's devastating, I'm speechless, I'm in shock I'm quite sad for our coach and his family because they're great ambassadors for this beautiful martial art," he said."I'm sure they're devastated and everyone who knows them."Nicole Foreman runs the childcare centre next door to the facility and said the centre had to close today, affecting more than 100 families."They rely on us for the kids while they go to work, so it's going to be hard for them to find somewhere for the kids to stay today," she said.Anyone who witnessed any suspicious activity in the area or has footage of the incident has been urged to contact CrimeStoppers on 1800 333 000.It follows the firebombing of another gym in Taylors Lakes on Tuesday night, which is also operated by Genesis Health Club.During that incident, two people working in the gym were told by masked offenders to leave the building before it was set alight.