Officials say several buildings were levelled and evacuations are underway.
Police told News 3 this is an “extremely active and fluid” incident.
Officials will be opening up an emergency operations system soon.
Reports came in after 7pm local time (1am BST) that a plume of smoke was seen from near Main and Bristol Street at the Barr House.
Andy Gay, a WMTV employee, said: “Our whole house shook”.
A local reporter said: “I can see the smoke from the near East side of Madison.”
Images of the scene show buildings completely destroyed and engulfed in flames.
The official Facebook page of the City of Sun Prairie wrote: “All residents are advised to avoid downtown Sun Prairie.
“Major gas explosion has shut down several blocks.”
Witnesses told local media that they felt the explosion a mile away.
So far there are no reports of injuries.
More to follow…