Kenyan media reports said that explosive devices, possibly powerful grenades, had been thrown at the buses from the side of the road.
The buses blew up in the Thika Road area close to the city centre.
Images shown on Kenyan television showed a red passenger bus with a large hole ripped out of its side, and a green bus with its roof and sided buckled by an explosion.
There was no official indication of how many people were killed or wounded in the attack, nor any claim of responsibility -- although Kenyan authorities are currently engaged in a major security crackdown targeting suspected supporters of neighbouring Somalia's Al-Qaeda-linked Shebab rebels.