Pol Major Kamthorn Ooycharoen said an electronic circuit link between a remote sensor unit and the bomb was being powered by six batteries but its switch had been turned off. The officer said the switch was probably turned off to prevent the circuit from being activated by other electromagnetic signals from mobile phones or remote-controlled toys.

The officer said the explosives had been assembled by a few experts in Thailand. He said the remote-control unit - similar to those used with toys - had a reception radius of up to 100 metres.

Police are looking into the lot numbers of TNT sticks and C4 explosive bars found in the car to determine which government agencies, military or police units they had been assigned to.

Kamthorn, who serves the Metropolitan Police Bureau's bomb-diffusion and disposal division, said he initially underestimated the bomb's potential as he was not aware the nine plastic containers for lubricants found in the car contained the powerful Ammonium Nitrate Fuel Oil (ANFO), which had a lethal radius of 1,000 metres. Buildings and the flyover in the area could have been destroyed completely if it had been detonated, he said.

Of his team's efforts before defusing the bomb, Khamthorn said the experts had set up barricades around the car and a large crowd was herded away to about 400 metres from where it was parked. Residents in the commercial buildings near the scene were told to shield themselves behind their buildings.

"After the bomb was defused, I was quite shocked to find ANFO inside the containers. Many people would have died if the bomb had exploded," he said.

Khamthorn said the bomb's circuit was completely set and ready to be detonated by remote control.

A witness to the bomb-disposal effort said she had seen a car parked on the footpath under Bang Phlat Intersection Bridge since 6am and a man wearing a cap was inside.

Jarupha Stanondchai, the owner of a building opposite where the car was parked, said she went to take her child to school and when she came back, it had disappeared.

"At about 8am, I saw the car was blocked by police officers near my building. I went out to see what was happening and the officers told me to go back because they had found a bomb hidden in the car."