Fifty people were killed on Wednesday when a bus travelling between the Kenyan capital Nairobi and the western city of Kisumu swerved off the road coming down a slope and rolled repeatedly, police said.
"The initial report we have is that the driver lost control of the vehicle," police officer James Mugera told Reuters at the scene.
Police and rescuers had counted 50 people dead, including seven children. "A few survivors" were taken to nearby hospitals, Mugera said.
President Uhuru Kenyatta sent his condolences to victims' families and wished those in hospital a quick recovery.
"I would like to remind drivers to always exercise caution to avoid such catastrophic incident," he said on Twitter.
Residents near where the crash occurred early on Wednesday morning heard screams and ran to the scene.
"I woke up when I heard the noise and came here with other people, we found passengers screaming and asking for help, we managed to save a few people who were alive but the others were already dead", said Jackson Kosgei.
Authorities are investigating the cause of the accident.
The Daily Nation newspaper quoted traffic commandant Samwel Kimaru as saying that 2,345 people have died in road crashes across Kenya in the past 10 months, an eight percent rise compared to last year.