Thailand upgraded security measures after two more blasts hit government buildings, the latest in a string of minor attacks since anti-government protests erupted this month.
The rallies mounted by red-shirted supporters of ousted premier Thaksin Shinawatra have been met with a heavy security deployment, including a lockdown on parliament that has triggered an opposition boycott.
Authorities said that the new blasts, which hit a provincial hall on Bangkok's northern outskirts and a government building west of the capital on Wednesday, had hit weak spots in the operation involving 50,000 personnel.
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"We have to adjust our operation to curtail the attacks," Deputy Prime Minister Suthep Thaugsuban told reporters.
'Government can't control situation'
"Intelligence reports say they want to create unrest to show that the government cannot control the situation, but I want to reassure the Thai people that the government is in control," he said.
The blasts, which follow eight other minor explosions since the protests began on March 14 to push for fresh elections, caused minor damage but no injuries.
Most of the incidents have involved grenades, but police investigating the provincial hall explosion, which left a small crater 20 centimetres (eight inches) deep, said they believed it was a bomb fuelled by TNT.
Metropolitan police commander Santhan Chayanont ordered police to increase the number of checkpoints and patrols, particularly on main roads and at key government facilities, and enforce searches for weapons and explosives.
Prime Minister Abhisit Vejjajiva on Tuesday extended a tough security law enacted for the demonstrations, applying it to Bangkok and nearby districts for an additional week as the "Red Shirts" vowed fresh action.
Lawmakers boycott parliament
Opposition Puea Thai lawmakers boycotted parliament for a second day Thursday to protest tight measures including concrete and razor-wire barricades surrounding the building and thousands of soldiers and police on duty.
The president of the upper house, Prasobsuk Boondej, also criticised the military presence as an over-reaction and said some senators had problems entering the building.
Abhisit, who has said the measures are aimed at preventing a repeat of the siege on parliament in 2008, rejected opposition allegations that the response was "a version of a coup".
Soldiers and police "have not come to seize legislative power, they want to ensure that we can exercise our legislative power, in fact they are protecting democracy," the premier said in parliament.
Further protests planned
The "Red Shirts" are planning a mass rally in Bangkok on Saturday, which they say will exceed in size the street parade they mounted last weekend when they drew 65,000 people and caused traffic congestion in parts of the city.
On Thursday they threatened to move their protest encampment, currently in the capital's old quarter, to the crowded downtown district if their opponents pursue plans to obtain legal orders to halt the rolling demonstrations.
The Thaksin loyalists, who have won headlines with stunts including donating their own blood to splash on Abhisit's home and offices, on Thursday shaved the heads of volunteers on their protest stage in a gesture of defiance.
