The fighting, which so far has claimed 30 lives and injured hundreds since anti-government Red Shirt protesters began camping in the capital on March 12, plunged Thailand deeper into political uncertainty, with both sides hardening their positions.
An Associated Press photographer saw protesters torching the bus near a downtown police station and retreating when a fire engine arrived to douse the flames.
The area is near a subway station that has been closed since Thursday, and close to the American and Japanese embassies, which were closed because of the unrest. The British and the Dutch embassy, which are in the vicinity, also were closed.
Lines of troops were seen moving toward the protesters on Wireless Road where the bus was set on fire, while gunshots were heard in the area. But it was not clear who was firing.
Gunshots rang out throughout the night and into the morning in central Bangkok. At daybreak, a group of protesters captured and vandalized two military water cannon trucks at the intersection of Sathorn and Rama IV roads in the heart of the business district. They ripped the cannon from its moorings and used its plastic barrel to shoot firecrackers from behind a sandbag bunker they had commandeered from soldiers.
The Red Shirt protesters, who have taken over an upscale 1-square-mile (3-square-kilometer) area in central Bangkok, vowed they will not give up until the government resigns and early elections are called.
"I'm not scared. We are here only to ask for democracy. Why are we facing violence?" Mukda Saelim, 39, a mushroom farmer from Chonburi province, said. "I don't have anything to fight them, but I'm not afraid. You asked if this is safe? It's not."
The Red Shirts believe Prime Minister Abhisit Vejjajiva's coalition government came to power illegitimately through manipulation of the courts and the backing of the powerful military. They are demanding he dissolve Parliament immediately and call new elections.
Chances of a compromise dimmed further after renegade army Maj. Gen. Khattiya Sawasdiphol, who is accused of creating a paramilitary force for the Red Shirts, was shot in the head Thursday evening. He was talking to reporters just inside the perimeter of the protesters' encampment in Saladeng when the bullet hit him.
He was taken to a hospital in a coma and was in critical condition. The hospital said his brain had swollen and that he was unlikely to survive.
It was not known who shot Khattiya, better known by the nickname Seh Daeng. But the Red Shirts blamed the government.
"This is illegal use of force ordered by Abhisit Vejjajiva," said Arisman Pongruengrong, a Red Shirt leader. "It is clear that there were no clashes at Saladeng, but Seh Daeng was shot by a government sniper. This is clearly a use of war weapons on the people."
My great-uncle is a retired Peace Corps volunteer, currently living in Bangkok. He says the government really wants to stop the rally now, as Thai schools open on Monday and there are 16 schools near the rally area.