The A30 is shut in both directions while emergency services deal with the blaze.
The westbound carriageway of the A30 near Fenny Bridges was closed due to flooding.
Police say five men have been arrested in relation to the incident on 2 November.
It happened in the early hours of Thursday 6 November, police say.
A team of volunteers are sweeping the streets to get the town back to normal.
Flaming tar barrels light up the streets of Ottery St Mary on Bonfire Night.
Up to 20,000 spectators watch people carrying flaming barrels through the town.
Hairdressers and tattooists are among those getting ready for one of the busiest times of the year.
A guide to timings, road closures and safety advice for the spectacular event.
