At least 30 people have sustained injuries.
The curfew has been imposed in Attanagalle, Watupitiwala and Nittambuwa until 6am SLT on Saturday.
There were about 80 civilians on board at the time of the attack, which took place during the evening rush-hour.
A bomb has been placed under a seat in the bus bound for Girulla from Nittambuwa, military officials said.
Director, Colombo General Hospital, Dr. Hector Weerasinghe told BBC Sandeshaya that at least 10 people have sustained serious injuries.
Most of the injured were admitted to Watupitivala.
Eight injured were transferred to Colombo.
Defense Affairs spokesman, Minister Keheliya Rambukwella, accused Tamil Tigers of planting a bomb in the bus.
It was clear that the LTTE was responsible "considering their past track record", Rambukwella told BBC Sinhala.
The LTTE has denied any involvement.
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