The security media centre said two people were killed and at least 20 injured. The injured were rushed to Katugastota and Kandy hospitals.
Tamil Tiger guerrillas have been blamed for a series of explosions against civilian targets in recent weeks.
On Friday morning a roadside bomb which peppered another bus with shrapnel killed 21 persons and injured 60 others in Katubedde, south of the Sri Lankan capital Colombo.
Earlier in the week a suspected Tamil Tiger bombed a rail track as a commuter train was passing through a suburb of the capital.