Arsonists climbed onto the roof of a gym and started a fire inside the building.
Police are investigating the shocking incident at The Warehouse Gym in Hindley which was discovered on Saturday morning.
Emergency services believe petrol was used to start the blaze and an object was dropped through the roof into the building.
It is thought the incident happened overnight between Friday and Saturday. No-one was in the building at the time.
Fire services were called to the gym at Hindley Business Centre on Platt Lane at around 9.10am on Saturday but found the fire had burnt itself out.
They immediately became suspicious of the circumstances and called in the police.
The extent of the damage caused by the blaze is currently unclear.
A message on The Warehouse Gym’s Facebook page said the facility would remain shut on Saturday “due to unforeseen circumstances”.
However, the post also stated the business hoped to re-open on Sunday.
Greater Manchester Police (GMP) have recorded the incident as arson and are now looking into it.
Anyone with information is asked to ring GMP on 101 or call independent charity Crimestoppers, anonymously, on 0800 555 111.