Four of the injured in the blast were admitted to Ramathibodi Hospital and one was hospitalised at a mobile medical unit of volunteer nurses near Makhawan Rangsan Bridge. The incident took place at about 1.30pm.
Pol Col Songpol Wathanachai, deputy chief of Metropolitan Police Division 1, said he had ordered local police to investigate. He believed it was the work of a third-hand party that wanted to create trouble to exacerbate the situation.
Police are to examine footage from security surveillance cameras in the area, he said.