It's believed arsonists were responsible for burning fifteen lavatories to the ground, after they were set up for a charity run in Worthing.
Four hundred athletes arriving at Hillbarn Rec for the Three Forts Challenge found the portable toilets, set up at Barn Playing Fields at Worthing College, completely destroyed early on Sunday.
But race organisers said replacements arrived ahead of a successful event supporting "Ferring Country Centre", for people with learning disabilites.
Worthing resident Valerie Turner told More Radio:
"It was very disappointing to arrive at Hillbarn Rec this morning, to set up for the annual Three Forts Challenge, and discover that the toilets ordered for the event had been burnt to the ground overnight, in an act of pure vandalism.
"Fortunately, our portable toilet supplier was able to deliver a back up facility.
"Despite this, the rain and the wind, the Three Forts Challenge was able to successfully go ahead and 400 runners were able to complete either the Marathon or Half Marathon route over the South Downs.
"This event raises several thousand pounds each year for Ferring Country Centre and it is sad that it was targeted in such a mindless way."
The cause supported by the event describes itself as a local, independent charity providing training and work experience for people with learning disabilities.
It operates a Riding Therapy Unit, Garden Centre, Rangers Café and Dales Farm, to help people achieve greater work and social skills.
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