The government casualty numbers and those given by the Tigers cannot be independently confirmed as Colombo bars journalists and rights groups from frontline areas.
The improvised explosive device was hidden in a flower bed on the central median of the main Colombo-Galle highway, said military spokesman Udaya Nanayakkara.
"One person was killed and six others were wounded and taken to hospital," a police officer in the area said.
He said the initial investigations suggested the man who was blown to pieces had tried to touch a package placed on the middle of the road.
A local hospital director said four of the wounded were children on their way to school. Eight shops on either side of the highway and two vehicles were badly damaged in the blast, witnesses said.
Police cordoned off the area as forensic experts were called in to help with the investigations.
The blast went off in the Wellawatte district of Colombo, where minority Tamil residents are concentrated, police said, adding