The explosion reportedly occurred behind a stage prepared for the Nopphamat Beauty Queen contest shortly after deputy governor Chawanin Wongsathitjirakal opened the annual Nakhon Phanom Loy Krathong Festival at 6pm.
The festival this year was held at Phanom Nakha Plaza by the Mekong River in Muang district.
Witnesses said the explosion at the fireworks preparation area created a "shockwave" that “pushed staffers back about five metres”, while several people nearby were injured by fireworks shrapnel and small fires.
All 15 were rushed to Nakhon Phanom Hospital. A news source said most suffered only small injuries and can return home after treatment. There were no reports of any critical injuries.
Muang Nakhon Phanom mayor Niwat Jiawiriyaboonya said he was not sure who brought the fireworks to the event, which had been planned as a fireworks-free festival.
“The province is deeply sorry for the incident and will take care of all medical expenses and any other compensation for the injured,” he said.
Fireworks are traditionally part of the Loy Krathong Festival to show respect to Phra Mae Khongkha, the goddess of water. They are, however, banned in airport areas, or “air safety zones”, along with lanterns, balloons, laser lights, drones and other items that might damage aircraft or pose a problem to pilots.