At least 23 civilians were killed and 80 injured in two separate attacks on civilian buses in Colombo and central Sri Lanka, a military spokesman Brigadier Udaya Nanayakkara said Friday.

In the latest incident Friday, a bomb exploded on a bus in central Sri Lanka killing two passengers and injuring 20 while earlier in the day a claymore mine ripped through a bus on the outskirts of Colombo killing 21 and injuring 60, he said.

The second explosion occurred at Polgolla, Kandy in central Sri Lanka, 125 kilometres east of the capital. In the first attack in Colombo, a mine was planted near a garbage dump by the roadside and had been triggered by a remote control near a university in Katubedda, 12 kilometres south of Colombo, police said.

The bus was carrying mainly office workers commuting to work at the time of the blast.

Military spokesman Brigadier Udaya Nanayakkara blamed the attack on separatist rebels with the Liberation Tigers of Tamil Eelam (LTTE).

'The mine is likely to have been placed in the early hours of Friday as we heard dogs barking in the area,' resident Anura Kumara said. 'In the morning, we found that at least seven dogs had been poisoned.'

A curfew has been imposed to facilitate a search while additional troops have been deployed to maintain security and arrest suspects. At least two university students were among those detained.

President Mahinda Rajapaksa said the attack was 'further proof of the LTTE's frenzy at its major military set backs in the north, and the loss its hold in the eastern province.'

President Rajapaksa urged the public to remain calm and not to 'give into deliberate provocations aimed at fanning communal hatred and violence and to remain vigilant against the forces forces of terror.'

Tamil rebels have stepped up attacks on civilian targets on the outskirts of Colombo as government troops have heightened their operations in the north against the ethnic Tamil rebels after vowing to defeat the LTTE by the end of the year.

The incident came two days after rebels set off explosives under a train on the outskirts of Colombo. The explosion injured 25 but did not cause major damage.

Nine civilians were killed and more than 80 were injured on May 26 when suspected rebels detonated a bomb on a passenger train at Dehiwala, 10 kilometres south of Colombo.

On Sunday, six civilians were killed in a claymore mine explosion in a rebel-controlled area in the north. Two more civilians were killed Thursday, also by a claymore mine, in a northern village in the rebel-controlled area.

Tamil rebels blamed the army's deep penetration unit for infiltrating the area and carrying out the attacks. The army has denied the claim.

Security in Colombo has been stepped up, and searches on vehicles entering the city have been intensified.

The police curfew imposed in the area of Friday's blast was expected to continue until at least late Friday.