Twenty people were killed and scores injured on Saturday when a powerful bomb, planted by suspected Tamil Tigers, ripped through a packed bus carrying civilians in a central Sri Lankan town.

The blast inside the private bus, heading to Anuradhapura from Kandy, occurred at 7.05 am local time at a bus stand in Dambulla town, located 148 km from the capital Colombo, the army said.

Defence sources believed that this was another "treacherous act of the Liberation Tigers of Tamil Eelam targeting innocent civilians," it said.

The blast left at least 20 people dead and scores injured, four of them seriously who were taken to the general hospital at Matale with head injuries.

Among the dead and wounded were many Buddhist devotees, who were on a pilgrimage to the sacred Anuradhapura city, Dambulla police said.