At least three people were killed and scores injured when a grenade was thrown into an annual fair in Umphang district of Tak province late on Friday night. Two teenagers, aged 16 and 17, were subsequently arrested.
The attack occurred at the Red Cross Doi Loyfa fair, said Pol Col Supakorn Phiphatphimpha, chief of Umphang police. The incident was reported to police about 11.30pm.
The grenade was thrown in front of a dance stage where residents and visitors were dancing to the music. The blast prompted people to flee for their own safety. Many fell to the ground in front of the stage, according to witnesses.
Pol Lt Gen Archayon Kraithong, spokesman for the Royal Thai Police Office, said that three people were killed and 48 others injured in the grenade attack.
The force of the explosion killed two people on the spot. The injured were rushed to Umphang Hospital after rescue workers provided first aid.
Some Thai media outlets reported the incident resulted in four fatalities.
Following the attack, police and soldiers arrested two male suspects: a Thai national and a member of the Karen National Union (KNU), said Pol Lt Gen Archayon. The KNU member, identified only as Jorithu, was previously arrested for drug offences in February this year.
During questioning, Jorithu admitted to carrying a shoulder bag containing the explosive to target a rival gang.
According to reports, the explosion was caused by an improvised explosive device (IED) and was linked to ongoing conflicts between teenage groups that began during the Songkran festival.
Manot Phoniam, chief of Umphang district, said the fair was held for seven nights, from Dec 8 to Dec 14. Teams of village volunteers, police and soldiers were arranged to beef up security during the event, said Mr Manot.
The event featured multiple show stages, and the attack took place at the traditional dance stage, he added.
About 8,000 to 9,000 people attended the annual Red Cross fair in this northern border district.