Five of them were severely injured in the attack at 00:15 am Tuesday morning.

Guards of the People's Alliance for Democracy said they saw an attack on the Don Mueang Tollway who sped off in a vehicle to the direction of Rangsit area after the attack.

The explosion caused turmoil inside the terminal where protesters were sleeping. They woke up and ran in panic.

Protesters and PAD guards said the grenade, believed to be an M79, pieced broke a glass window pane of the terminal and exploded inside the terminal.

Two injured protesters were rushed to the General Hospital, 7 to Bhumibhol Hospital and 14 to Mongkut Watana Hospital.

Ronnachai Chaisri, 29, from Songkhla province's Sabayoi district, died on his way to the Bhumiphol hospital.

Three severely-injured protesters at the Bhumiphol are Prasong Kongprasutr, 55, Somsak Chaisuwan 46 and Wijai Wijan, 43. They were hit by bomb shrapnel on their abdomen and received urgent surgeries to save their life.

Two injured protesters at the Central General are in critical condition. They are Phanchawee Suphatpong, 66, and Piyachart Daengpuangpaibul, 30. Phanchawee was hit at her headd while Piyachart at his knee.

Kriangkrai Piayu, a guard from Nakhon Sawan, said he saw a man waling on the Don Mueang Tollway when he heard the explosion.

The man then sped off in a vehicle.

"The attacker is very inhumane to attack the innocent people. He should have attacked us, guards, instead," Kriangkrai said.

A protester said he saw a flash of light from the direction of the tollway and a window pane was smashed and then the explosion followed.

The attack came after the PAD deserted and ended its occupation of the Government House for security reasons, saying it would be safer for protesters to rally at the Don Mueang and Suvarnabhumi airports