Organisers of Sunday's Conwy Half Marathon warned runner they would be disqualified if they dropped litter during the race. About 3,000 runners took part.
The measures had been introduced as plastic was becoming an "increasing problem", as reported by the bbc.
Runners would also be "taken off the results if seen discarding their rubbish outside of a water stop or not with a marshal", according to The Run Wales website.
Organisers have not yet said if anybody had been disqualified following the race, which started in front of the town's castle at 10:00 GMT.
Earlier this year, The London Marathon trialled several measures to reduce litter after clearing 47,000 plastic bottles from the streets in 2018. These included using compostable cups rather than plastic bottles at some stations along the route, and using plastic bottles made wholly or partly from recycled plastic.