No casualties were reported due to the explosion, Colombo National Hospital sources said, adding that 4 people have been admitted to the hospital with minor injuries.

Passengers have immediately got-off the bus and reached safety distance after bomb fear raised of an unattended parcel inside the bus, Defence Ministry website said citing civilian sources.

"This is a heavy traffic hour and many school children are on the roads", the website said adding that the alertness of the bus conductor and driver have saved the day for many.