A joint force of policemen and bomb disposal experts, led by Pol Col Krisda Kaewchandee, the deputy Narathiwat police chief, checked the two blast sites near the checkpoint. No one was hurt in the attack.
The first grenade exploded at the front gate of a building housing a village waterworks system, leaving a hole about 10 centimetres deep and 20cm in diameter.
The second landed under a mango tree about 30m from the first spot, creating a crater of about the same size.
Authorities were immediately informed about the night attacks, but decided to visit the scene the following day, fearing that the attack was a ploy to draw them into a trap.
Police said the grenade attacks were carried out by two men who arrived on a motorcycle along the Tak Bai-Narathiwat road.
The two stopped about 300m from the checkpoint near Ban Sala Chuak in tambon Sala Mai. One of them threw the grenades at the checkpoint but missed the target, police said.
Officials believe militants were behind the attack in an attempt to mark the eighth anniversary of the Tak Bai tragedy.
The Tak Bai massacre took place on Oct 25, 2004, when the military cracked down on protesters in front of Tak Bai Police Station. Eighty-five people died, of whom 78 suffocated while being transported to detention centres.
Meanwhile, three members of the separatist Runda Kumpulan Kecil (RKK) turned themselves in to authorities at a government centre in tambon Kok Tian of Narathiwat's Muang district yesterday morning.
The three were identified as Arduenan Tuanbuesa, 33, Ari Jehsoh, 28, and Mohammad Ritanuo Tarasi, 35. Mr Arduenan was accused of shooting at a train carriage last year. Mr Ari is suspected of gunning down Somphong Saui in June last year and a military ranger informant this year. Mr Mohammad is a suspect in a security-related case.
Authorities said the suspects had promised to stop engaging in violence and to cooperate with officials to halt the unrest in the South. They had also vowed to end their associations with insurgent networks, officials said.
Pol Col Krisada Kaewchandee, deputy commissioner of Narathiwat police, promised to facilitate the legal process for the suspects. They will be allowed bail for 200,000 baht.
Narathiwat governor Apinant Suethanuwong promised to ensure the safety of their families after their surrender.