Seven fire engines were called to a fire that threatened to destroy Lancaster’s St Nicholas Arcades.
Crews arrived at the scene at 8.50pm on Wednesday, and found the roof of Poundland in Lancaster Gate on fire.
Flames could be seen coming from behind the former Woolworths building and spreading into Penny Street, Market Street and the city centre.
They spent two hours at the scene and managed to stop flames destroying any shop.
Lancaster Fire Crew Manager Scott Lee said: “We believe it was deliberate, some people got up on the roof, we don’t know how, spilt some oil of some sort and set it on fire.
“When we arrived town was pretty quiet, we got up onto the roof through Diggles, luckily the lady was still there and it took us about 30 minutes to extinguish the fire.”
There were no casualties and an investigation is underway.