At least five people were killed when a grenade tore through a minibus in Nairobi's Somali-dominated Eastleigh neighbourhood on Sunday, a Reuters witness and police said.
"I saw bodies ripped apart," the Reuters photographer said.
The force of the explosion tore apart the vehicle's roof and seats and shattered the window of a nearby cafe. Two other cars were damaged, the photographer said.
Kenya has suffered a string of deadly attacks in its capital Nairobi, the southern port city of Mombasa as well as the eastern garrison town of Garissa over the past year.
The attacks have been blamed on Somali militants and their sympathisers in retaliation for Kenya's decision to send troops into Somalia last year to drive out al Qaeda-linked militants which Nairobi has blamed for attacks on its territory.
African Union peacekeepers, led by Kenyan forces, drove al Shabaab militants out of their last major urban stronghold of Kismayu in southern Somalia seven weeks ago.
Nairobi regional police commander Moses Ombati said the grenade had been thrown into the minibus, commonly referred to as matatus in Kenya.
Ombati confirmed five people had been killed, while the Kenya Red Cross said on its Twitter account that seven people had died and 24 people had been taken to hospital.
There was no immediate claim of responsibility for the attack.
In July, masked assailants launched simultaneous gun and grenade attacks on two churches in Garissa, killing at least 17 people.
Armed cattle raiders killed at least 32 Kenyan police officers in a military-style ambush last weekend. That attack exposed how ill-equipped Kenya's police force is, at a time when they are facing new challenges, including a presidential election next March.