The bombing, which took place behind the stage near the Pratunam intersection at about 8.45pm, injured five men and a woman. Three of the injured were rushed to Ramathibodi Hospital, two others to Chulalongkorn Hospital and one to Phyathai Hospital.
Five of them sustained minor injuries and were allowed to leave the hospitals.
A 40-year-old man, who sustained a back injury, was still under medical supervision at Phyathai Hospital.
Since Nov 30 last year, 16 people were killed and 695 others injured in attacks and clashes in connection with the ongoing anti-government protests, led by the People's Democratic Reform Committee (PDRC). Twenty-four of the injured are still being treated in hospitals.
Suthep Thaugsuban, secretary-general of the PDRC and former Democrat Party powerbroker, told the crowd gathered to hear him speak on Friday night that the explosion had been caused by a bomb thrown at a car.
Meanwhile, police on Saturday morning were investigating the scene of the bombing, as motorists were able to use the Pratunam intersection.