He said the explosion occurred at 7:05 pm before the lifting of the curfew. The curfew was lifted at 9:30 pm Friday night.
Police found that the bomb was a Russian-made RDG5 grenade. Somyos said someone might discard the grenade there and a car might accidentally run over it.
He said it was also possible that the explosion was caused by a fight between local rival gangs of teenagers.
But police would not rule out political motivation and would check the security cameras in the area to see what happened.
He warned that curfew might be re-imposed if violence continued in the capital.