The government said the bomb was the work of Tamil Tiger rebels, though they denied the attack.
Sri Lanka’s air force responded by bombing Tiger headquarters near Kilinochchi.
“They have done this in the past and disowned responsibility to avoid becoming international pariahs,” Sri Lanka’s Media Minister Anura Priyadarshana Yapa said, according to CNN. “But the world now knows. We have no doubt they were responsible. There are no others who use Claymore mines against the civilians and the military.”
Officials believe the explosion was caused by two landmines attached to a tree and detonated remotely, the Associated Press reported. The style of attack is a common Tiger tactic, the news service said.
“The blasts hit the middle of the bus and it overturned, trapping passengers,” one witness said.
“I lost my entire family of 13,” another told the AP. “My wife, mother, two children … are dead.”
Many of the victims on the bus — mainly ethnic Sinhalese — were commuters and school children. Others were family members of a policeman killed the day before traveling to his funeral, officials reported.
The leader of the Tamil Tigers, officially called the Liberation Tigers of Tamil Eelam, condemned the attack as the work of a breakaway faction.
“The Liberation Tigers condemn the attack on civilians in strongest possible terms,” the pro-rebel Web site TamilNet quoted Seevaratnam Puleedevan as saying.
Thursday’s violence, the worst since a 2002 cease-fire brokered by Norway nominally ended Sri Lanka’s 20-year civil war, may bring the island nation closer to a second round of battle.
The attack caps nearly a year of renewed fighting between the government and the Tamil Tigers, which started in August 2005 with the assassination of Sri Lanka’s Foreign Minister Lakshman Kadirgamar.
The Tamil Tigers have been fighting since 1983 for a separate homeland in the north and east of Sri Lanka. They claim the majority Sinhalese government oppresses the country’s 3.2 million minority Tamils.
The Tigers control large parts of the country and have set up a de facto state.
The Sri Lankan government refuses to divide the nation and has ruled out an autonomous region for the Tamils.
Both sides deny responsibility for the renewed violence and have said a return to civil war could be disastrous.
An estimated 65,000 Sri Lankans have died in fighting since 1983.