He declined to say what evidence pointed to former politicians less than five months after a bloodless coup ousted Thaksin, the twice-elected prime minister.
Involvement of Muslim fighters, whose insurgency has cost more than 1,800 lives over the past three years, was a "very remote possibility", he said.
"We could not at this stage pinpoint which particular group was involved," said Surayud, installed after the September 19 coup.
"Briefs from various intelligence agencies, based on evidence available, show that they came from groups that have lost political powers.
|A Thai policeman, right, inspects the damage |
at a shop caused by one of the bombs
"These were not just the previous government, but include all those which have lost power in the past."
Since Thaksin was ousted and his administration scrutinised by corruption busters, several schools have been torched outside Bangkok and martial law kept in several areas because of what the army calls "undercurrents".
But leading members of Thaksin's Thai Rak Thai (Thais Love Thais) party have denied it is involved in any form of violent resistance to the coup.
Thaksin's lawyer, Noppadon Pattama, said the former leader, believed to be in China, was the victim of "baseless accusations, a smear campaign" by a government which has refused to allow his client to return home to defend himself against corruption allegations.
Meanwhile, police and soldiers mounted a major security operation. Australia said it feared more attacks and joined Britain and the US in urging their citizens to stay indoors.
General Sonthi Boonyaratglin, the coup leader and Thailand's first Muslim army chief who cut short his Hajj and returned to Bangkok on Monday, ruled out any links to foreign groups such as al-Qaeda.
Police and soldiers set up 6,000 checkpoints across the city of nine million people and its suburbs after what General Achiravit Supanpasat, deputy national police chief, said was the worst attack in Bangkok in his 40-year career.