A car loaded with explosives that was intercepted by police early yesterday morning was possibly to be used in an attempt to kill caretaker Prime Minister Thaksin Shinawatra while he was travelling in his motorcade, National Intelligence Agency director Jumphol Manmai said yesterday.

The police general said he learned that the bomb, containing 14.23 pounds (6.5 kilograms) of a combination of TNT and C-4 composite explosive as well as a type of 64.57 blasting agent, was ready to go off. He said he did not know who was behind the possible assassination plot.

Metropolitan Police Bureau commissioner Lt-General Wiroj Jantharangsee said the explosives in the sedan were completely assembled, equipped with a remote unit sensor and ready to be detonated, and that it would have a radius of impact of around one kilometre.

Five uniformed police officers pulled over the silver Daewoo sedan at the foot of a flyover on Borromratchonnani Road near Bang Phlad Intersection and arrested the driver, Army Lieutenant Thawatchai Klinchana, who said he was paid Bt200 by a man to drive it to an area called Suan Oy.

Thawatchai, initially charged with having explosives in his possession, was later brought to the Crime Suppression Division compound for questioning, with two military officers present, after police searched his official residence near the First Military Precinct headquarters, where police found and seized two licence plates and a personal computer.

A police team later searched Thawatchai's own home in Nonthaburi's Bang Bua Thong district and found no illegal items. A Thai Rak Thai Party blazer was found there.

A police barricade was set up around the car and a large crowd was herded away to a 400-metre radius from where it was parked. A team of police ordnance experts were called in to examine the car. They neutralised some of the explosives on the front passenger seat by using a high-powered water gun to disassemble what appeared to be cloth bags with something inside.

Police discovered later that the sedan's licence plate Thor Santhan-Chor Ching 3085 BKK was registered to a woman and had already been classified void. An investigation was underway to find out how the invalid licence plates came to be attached to the car and what the car's body serial number, which had been scratched out, was.

At a press conference later on, the officer displayed a number of items he said were retrieved from the sedan. Explosives-related items included TNT sticks weighing 10.73 pounds in total, a number of C-4 explosive sticks weighing 3.5 pounds and a combination of blasting agents identified as Anfo (ammonium nitrate-fuel oil) contained in 13 lubricant plastic cans.

Wiroj said such types of explosives were the same as those used in terror bomb attacks in the restive South by insurgents, and that the impact would have been massive. "Nobody within the lethal radius of 30-40 metres would have survived and buildings within the range would be destroyed completely, including the flyover," he said.

The officer said it was possible the car was intended to attack the motorcade of Thaksin, which regularly uses the route to take him from his home in nearby Charansanitwong Soi 69 to Government House.