At least 17 people were killed and 45 wounded in grenade attacks at two churches in Garissa.
The town in northern Kenya is an important military base from where ground forces have been deployed into Somalia to fight al Qaeda-linked insurgents.
Kenya has been struck by a series of blasts since it sent troops across the border in October to try to crush al Shabaab militants.
The capital and other parts of Kenya have suffered a string of grenade attacks.
A blast hit a nightclub in the port of Mombasa last Sunday, killing one man, a day after the US embassy warned of an imminent attack on the city.
Garissa is situated about 100 km (60 miles) from Kenya's Dadaab refugee camp, where gunmen kidnapped four aid workers and killed a driver on Friday.