A grenade attack late on Saturday near a stage set up for rallies in front of Government House wounded at least 50 protesters, who have occupied the prime minister's office in Bangkok since August.
"The grenade was launched just after midnight, 300 metres from the stage in the Government House compound," Suriyasai Katasiya, a spokesman for the PAD protest movement, told local Channel Three television.
"Protesters have returned to their positions, they are not scared."
Two other blasts hit an anti-government television station, without causing any injury, and an explosive device also detonated on the road near the main entrance to Don Muang airport, Suriyasai said. Two people were injured in the latter explosion.
A series of grenade attacks on the PAD camp at Government House last week killed two protesters. No one has claimed responsibility for any of the incidents.
Heat on prime minister
Thailand's political crisis has deepened since the PAD began a "final battle" on Monday to unseat Somchai Wongsawat, the Thai prime minister, who they accuse of being a pawn of former leader Thaksin Shinawatra, his brother-in-law.
Pressure is building on the army to oust Somchai, as they did in 2006, after he rejected military calls to quit this week.
In a televised address on Thursday night, he said the PAD members barricaded at the airports were doing massive damage to the economy, but he would avoid violence to end the protests.
"Don't worry. Officials will use gentle measures to deal with them," Somchai said, inviting rights groups and journalists to monitor the imposition of emergency rule at the two airports.
He took a tougher line with his police chief, demoting General Patcharawat Wongsuwan to an inactive post on Friday.
While no official reason was given, local newspapers said Patcharawat had been sacked for refusing to send riot police in to end the protest.
Commanders on the ground said that they would not yet try to evict by force the thousands of protesters at Suvarnabhumi and Don Muang airports.
PAD supporters, for their part, deflated the tyres of ambulances and police vehicles at a police checkpoint.
Several vehicles were left stranded in the middle of the road.
The PAD say they are ready for a prolonged siege, with their "security guards" armed with clubs, sticks and golf clubs, and dug in behind a series of barricades of fire trucks, razor wire, car tyres and luggage trolleys.
Chamlong Srimuang, PAD co-leader and a retired general, told supporters on Saturday not to go to Suvarnabhumi as there were enough people there and instead go to Government House, where the protests started months ago.
The airport sit-ins have forced hundreds of flights to be cancelled, stranded thousands of foreign tourists and grounded millions of dollars of air cargo.