BANGKOK, THAILAND - A BOMB exploded on Thursday at Thailand's main government complex as the prime minister chaired a meeting, but it caused no casualties and was probably left behind during recent protests, officials said.

The small device detonated in the Government House compound in Bangkok as Prime Minister Abhisit Vejjajiva held a national security meeting attended by the defence minister, army chief and police chief.

The bomb went off as a worker was closing a manhole during security checks at the building, ahead of a national children's day celebration on Saturday, said Mr Pongsak Siriwong, director of the Household and Vehicle Division.

Police suspected the bomb had been left by demonstrators from the royalist People's Alliance for Democracy (PAD), thousands of whom occupied the complex in August last year in protest at the previous government.

The PAD left Government House in December after a court verdict led to the collapse of the old government, which was loyal to ousted former premier Thaksin Shinawatra, the protest movement's arch foe.

Police who cleared the compound afterwards said they had found explosives and several kinds of weapons.

Two PAD protesters were killed in grenade attacks on their protest camp at Government House last year. No one has been charged in connection with the attacks.

Mr Abhisit, from the anti-Thaksin Democrat Party, was elected as prime minister in December after winning the support of defectors from the ex-ruling People Power Party as well as the party's former coalition partners.