COLOMBO, Sri Lanka (CNN) -- At least 22 people were killed Friday in two bus bombings in Sri Lanka.

Sri Lankan security officials inspect the bus that was the target of the roadside bomb attack.

Initially, police said 21 commuters were killed and more than 53 wounded when an explosion hit a crowded bus in a town south of the capital of Sri Lanka on Friday, police said.

Police accused Tamil Tiger rebels of carrying out the attack in Moratuwa, a southern suburb of Colombo. Police said they also discovered and defused a Claymore mine in the same place, preventing another explosion.

A few hours later a bomb explosion in a bus outside Sri Lanka's hill capital of Kandy killing one person, police said.

The incident took place at Polgolla, some eight kilometres from the outskirts of Kandy where the bus was headed from the hill country town of Matale.

Police also blamed Tamil Tiger rebels for that explosion.

At Moratuwa, police and soldiers sealed off an area around the scene and searched for suspects. Authorities also intensified security in Colombo.

Initial reports said an explosive device was placed on the ground and detonated with a remote control device, police said. In an adjoining town, another bomb was discovered and diffused immediately, they added. Watch medics treat bus blast survivors »

The explosion came as rebel bomb attacks on civilian targets in Colombo and its suburbs have showed a marked increase. Pro-rebel web sites have accused Sri Lankan security forces of attacks on civilians in Wanni, the northern region dominated by them.

On Wednesday, a packed train heading to Colombo from the southern suburb of Panadura narrowly escaped severe damage when an explosion hit the track. The blast injured 23 commuters.

On May 26, an explosion on the same southern railway line killed nine and injured 84.

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