A Kenyan policeman stands in front of the wreckage of a bus at the site of a bomb blast in Nairobi on May 4, 2014
The bombings came the day after twin attacks in the restive port city of Mombasa, including a grenade attack on a bus, which killed four, and a bombing outside a luxury beach hotel.
Kenya's Disaster Operation Centre said the bus bombings had killed two and wounded 62, with 20 of them in a critical condition.
"Two people have died while being taken to hospital," a police official also told an AFP reporter at the scene.
There was no immediate claim of responsibility for the attacks - although Kenyan authorities are currently engaged in a major security crackdown on suspected supporters of neighbouring Somalia's al-Qaeda-linked Shebab rebels.
Kenyan media reports said bombs appeared to have been planted on the buses, although there were unconfirmed reports that powerful grenades may have been thrown at them from the side of the road.
The buses blew up along the Thika Road highway, an area around eight kilometres (five miles) northwest of Nairobi's city centre.
An AFP reporter at the scene saw a red passenger bus with a large hole in its side, and with the ripped panels spattered in blood. Kenyan media also showed images of a green bus with its roof and sides buckled by an explosion.
The Nation newspaper said the 45-seater buses were almost full when the blasts occurred.
Both Nairobi and Muslim-majority Mombasa, a port city that is one of the main gateways to east Africa as well as a popular tourist destination, have been hit by sporadic unrest in recent months.
Kenya has been targeted by Shebab since sending troops to war-torn Somalia in 2011. Kenyan soldiers are still posted in southern Somalia as part of an African Union force supporting the country's fragile internationally-backed government.
The Islamist group claimed responsibility for the high-profile attack on Nairobi's Westgate shopping mall last year in which at least 67 people were killed.
In March two people were arrested in Mombasa along with a car expertly packed with explosives. Intelligence sources say they believe the car was rigged in Somalia and driven into Kenya for a high-profile bombing.
Nairobi and Mombasa have seen a string of smaller bombings and shootings blamed on Islamists, pushing national security to the top of the agenda in the east African nation - which once identified itself as a bastion of stability in the region.
Kenyan police have also been under fire for an ongoing crackdown in Nairobi which has seen thousands of people detained - most of them Somalis or ethnic Somalis.
The operation has focussed on Nairobi's main Somali district Eastleigh, and residents have accused police of indiscriminately arresting people of Somali origin.
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