The fire in a flat above the former Nutritional Centre closed the main A420 though Kingswood town centre during the evening rush hour.
Avon Fire & Rescue service told the police they believed the fire was suspicious. No one was injured.
Patrolling officers stopped two men nearby and arrested them on suspicion of arson within 25 minutes of the call. A 23-year-old man remains in police custody today, while a second man aged 20 was released on police bail.
Anyone with any information which could help police inquiries into the incident is asked to call South Gloucestershire CID on 101 or Crimestoppers on 0800 555 111. They never ask your name or trace your call.