Firefighters were called to the scene after a mattress caught fire in a small building in the grounds of the Usher Gallery.
Lincolnshire Fire & Rescue were called at 12.19pm on Monday, October 8 and sent a crew from Lincoln North, who extinguished the fire and made the area safe.
The cause of the fire was discarded smoking materials and firefighters have now left the scene.
A spokesperson for Lincolnshire Fire & Rescue said: “On arrival it was discovered that it was a mattress on fire in the grounds of the Usher Gallery and the crew extinguished the fire and made things safe. The cause was carelessly discarded smoking materials.”
A passer by who witnessed the incident told The Lincolnite: “It looked like a pile of carnage. Many people were having lunch on the grassy areas and a big group of young people were standing at the site. One lad threw a bottle of water on it.”