President Mahinda Rajapakse said Friday's blast was the work of the rebel Liberation Tigers of Tamil Eelam (LTTE) guerrillas who are resisting a major military campaign in the north of the island.
The president in a statement warned that the rebels could resort to further indiscriminate attacks and urged residents to exercise more caution.
A powerful time-bomb ripped through an overcrowded bus, blowing off its roof, as it pulled out of the Piliyandala terminal into rush-hour traffic on Friday on the outskirts of the capital.
"Two more passengers died in hospital and the number of people remaining in hospital this morning is 64," a police spokesman said.
Grief-stricken relatives gathered outside two hospitals to claim the dead from the explosion on the bus, which was jammed with office workers and schoolchildren returning from private classes.
Among those killed were a 10-year-old boy, a Buddhist monk and eight women.
"The president deplored the LTTE bomb attack on a packed