A pro-monarchy supporter caught with a pistol and bullets at the rally site on Tuesday night told police he carried the gun for self-defence.
Kasidit Leelamuktanan, 35, was detained by soldiers. They seized a gun and 10 bullets from him.
During police interrogation, Mr Kasidit admitted he took part in the pro-monarchy demonstration on Tuesday, but said he had the pistol with him only for self-defence.
As clashes between pro-democracy demonstrators and pro-monarchy supporters erupted on Tuesday evening, he decided to hide the gun and bullets near a fence. He feared it could cause problems for him.
When the situation calmed down, he went back to where he had hidden his gun.
Soldiers noticed him rummaging around for something. They moved in to see what he was doing, and found the gun. They held the man and called police.
