Four people were Saturday morning killed in an accident in Njuri, Tharaka Nithi on the Meru-Nairobi Highway.
Police said about 20 others were injured in the 5 am incident.
According to police and survivors, a school bus was ferrying 46 members of Thau Methodist Church from Tigania East to a wedding in Karen, Nairobi when the accident happened.
Officials said it took longer to remove some of the victims who were trapped in the wreckage.
The church chairman Joseph Limuli said the driver lost control of the bus after one of its tyres burst.
He said they had left the village at around 2 am.
Police arrived at the scene and helped to coordinate the rescue mission with locals.
Eastern police boss Rono Bunei said most of the injured were taken to local hospitals.
He said they are investigating the incident.
Accidents are common on the route and most of them are fatal. Many are nursing wounds from the accidents.
Bunei said they are embarking on a campaign on road safety in the region to address the menace.