The report said at least six people were injured by the blast, which took place behind the stage near the Pratunam intersection.
Three of the injured were rushed to Ramathibodi Hospital, two others to Chulalongkorn Hospital and one to Phyathai Hospital. Many cars were damaged by the blast.
Suthep Thaugsuban, leader of the People's democratic Reform Committee (PDRC), told the crowd gathered to hear him speak that the explosion had been caused by a bomb thrown at a car.
Police were investigating and telling motorists to avoid using the route.