The explosion occurred at 7.35 AM in Moratuwa, 12 km from Colombo, a military spokesman said and blamed the LTTE for the attack.

"Twenty-one passengers were killed in the explosion.

Seven of the dead were women," he said.

The blast is the latest in the string of attacks targeting civilians since Colombo scrapped a tattered 2004 ceasefire with the Liberation Tigers of Tamil Eelam.

The roadside bomb was placed between Shailabimbaramya Buddhist temple and the University of Moratuwa and destroyed the public transport bus, defence sources said.

The bus was travelling from Kottawa to Mount Lavinia, about 5 kms from Colombo.

Meanwhile, hospital sources said out of the 21 people killed, eight were women.

It said the the injured have been rushed to the nearby Teaching Hospital and Base Hospital.

A few victims who suffered severe injuries have been transferred to the National hospital at Colombo.