At least four people, including two military personnel were killed and 23 others injured when a powerful bomb went off in Colombo on Wednesday, targeting a bus carrying army personnel, defence and hospital sources said.

Director Accidental Service of the Colombo National Hospital Hector Weerasinghe said at least 25 wounded people from the blast site were admitted to the hospital, out of which three people succumbed to their injuries.

Of the wounded people there were few army soldiers and the rest of them are civilians, Dr Weerasinghe said, adding that most of the seriously wounded people were being treated in the critical care section.

According to latest details, at least 10 military personnel, travelling in the bus, were injured.

The blast has occurred at a heavily guarded and crowded Slave Island area, a few hundred meters away from Sri Lankan Army and Air Force headquarters. The area had been cordoned and security beefed up.

The blast has occurred exactly a day after a Sri Lankan ethnic Tamil lawmaker from the main opposition United National Party (UNP) T Maheshwaran and his personal bodyguard were killed and 12 others wounded when an unknown gunman sprayed bullets at the MP inside a crowded Hindu temple in Colombo.

The defence officials said the alleged Tamil gunman, who was wounded in retaliatory fire from the MP's personal security officer, has been arrested and further investigation in this regard was under way.