A car bomb went off at the foot of the first Thai-Myanmar Friendship Bridge on the Myawaddy side of the border opposite Mae Sot district late on Saturday night, a security source said on Sunday.
The explosion was clearly heard in Mae Sot, he said.
The bomb is believed to have been placed in a vehicle left in front of the gate of the Myawaddy border checkpoint at the foot of the first Thai-Myanmar Friendship Bridge by an anti-government force.
There was a power cut in the immediate area after the blast which was followed by the sound of about 30 gunshots.
A four-storey building near the bridge on the Myawaddy side was gutted by the fire while several buildings and vehicles in the area were damaged. There were no reports of casualties, he said.
Meanwhile, in Mae Sot, village security units, a rapid-deployment company of the Naresuan Force and Border Patrol Police Company 346 stepped up patrols along the border to ensure security.
One unit spotted numerous Myanmar people who had fled across the border to the Thai side of the Moei River before the explosion and managed to round up 24 men and 25 women.
The 49 said they had illegally crossed the border to Moo 2, a riverside village in tambon Tha Sai Luat, and were waiting for job agents to pick them up.
They claimed to have paid 20,000 baht each to job brokers.
All are expected to be deported after being processed at Mae Sot police station.