Three children were seriously injured after a bus they were travelling in overturned near Capital Centre on Mombasa road.
At least 13 other casualties have been reported from Wednesday morning incident, and seven are said to be in critical condition.
They were rushed to the Kenyatta National Hospital by the Kenya Red Cross.
The driver of the bus is said to have been speeding, after which he lost control of the vehicle.
Police arrested the driver who will be charged with reckless driving.
The bus is said to belong to Trinity Transporters.