Two engines from Preston attended the fire within a sheltered area of the Syke Street Car Park, behind Evoque nightclub, at around 12.30pm today (Wednesday, May 29).
Crews were alerted to the blaze after smoke activated a fire alarm inside the Church Street night club.
On arrival, fire crews found a burning mattress, a smouldering sleeping bag and other fire-damaged belongings scattered about the car park.
The sheltered area of the car park is often used by the city's homeless and it is feared that a rough sleeper's possessions might have been deliberately targeted.
The cause of the fire is still under investigation, but it is understood to have been started deliberately.
Lancashire Fire and Rescue Service confirmed that the suspicious fire has been reported to police.
Fire crews were at the scene for about an hour and used a hose reel and a thermal imaging camera to extinguish the flames.
Lancashire Police have been approached for comment.