Twenty Jubilee Party delegates were injured in an accident at Kaburenguk on Eldoret-Malaba highway, a few meters from River Nzoia.
They were among hundreds who had travelled from Bungoma to attend the party's official launch in Nairobi on Saturday.
Bungoma West DCI Paul Mugo said they left Nairobi in a bus belonging to St Mary's Kibabii Boys High School that night.
Mugo said the driver of the bus lost control and rammed into an oncoming trailer.
He said the delegates were rescued by police officers who were on patrol and members of the public.
Webuye county hospital medical superintendent Patrick Mutoro said 15 patients were admitted and the rest treated and discharged.
He said those admitted were in stable condition.
