A man who was using it at the time was slightly injured by broken glass.

Prapat Muangsongchan, 22, said he had noticed flames behind an electricity pole just before the bomb explode about 9:30 pm.

The phone booth is located on the corner of Rajvithi Road Soi 24.

Acting national police chief Pol Gen Seripisut Temiyavej, Metropolitan police chief Pol Lt Gen Adisorn Nonsee, his deputy Pol Maj Gen Krissada PhanKongchuen, and Bangkok Governor Apirak Kosayothin rushed to the scene shortly after the explosion.

Adisorn said police set up road checkpoints around the palace on around-the-clock basis.

He said police dogs were also deployed to check the areas and government office around the palace.

He said the Metropolitan Police would hold a joint meeting with the Bangkok Metropolitan Administration and the First Army Area at 9:30 am Sunday.

Adisorn said police bomb experts found that the explosives used in the bomb did not have high destructive power so the bomb was not aimed to cause high damages or loss of lives.

He said the bomb had only explosives without shrapnel and it was detonated by a timing device.

"We believe it was an attempt to create disturbance," the metropolitan police chief said.

He said Prapat and a guard of the palace, who saw Prapat making the phone call, were the only two witnesses available but no one saw the person who put the bomb there.

First Army Area Commander Lt Gen Prayuth Chan-ocha said the military could not yet analyse the motives behind the explosion as more information was needed.

He said soldiers in plainclothes and in uniform had been deployed to help police keep security around Bangkok. But the number of deployed soldiers would be doubled following the latest explosion.

Apirak said the explosion was like the bombing at the Major Ratchayothin departmentstore.

"We hope the people will not be panicked because those behind the bomb only want to create situation," Apirak said.

He said the BMA, police and soldiers were cooperating in deploying officials to check risk areas all the time.

Apirak also called on the public to immediately alert officials once they notice some irregular things.

The Bangkok governor said he would convene a meeting of directors of all 50 Bangkok districts to order them to step security in the capital.

The Nation