A crush at a playground in the southwestern Nigerian city of Ibadan killed 35 children and sent six others to hospital, police said.
The event was advertised as free admission and attracted large crowds, but the sheer number of people made it difficult for them to enter.
Residents said more than 5,000 children had gathered at the venue and a crowding incident occurred when the main organizers arrived for the event to begin.
The sequence of events leading up to the squeeze has not yet been revealed.
Police said they had arrested eight people behind the event, including the main organizer and well-known figure in the city, Naomi Silekunola.
The Oyo State Government says victims of a car accident at Islamic High School in Basoron District have been taken to hospitals across Ibadan for treatment.
The statement said “many people were killed and injured”.
Authorities urged parents concerned about their children’s whereabouts to go to the city’s hospitals for tests.