BANGKOK (Reuters) - Thai troops and armored vehicles massed in Bangkok’s main business district on Wednesday and fired warning shots into the air ahead of a possible operation to evict anti-government protesters from their fortified encampment.

Troops called on protesters and civilians to leave their fortified encampment in central Bangkok and surrounding areas immediately.

“Please leave the site immediately. Officials are about to conduct an operation,” the military said on a loudspeaker, according to a Channel 9 television reporter.

The military buildup comes a day after the collapse of a proposal for talks aimed at ending five days of chaotic street fighting that descended into urban warfare in which 39 people were killed and nearly 300 wounded

Soldiers sprayed a water cannon at a wall of tires at the entrance to the protest encampment after protesters doused them with fuel to prevent them from being ignited.

A leader of the red-shirted protesters called on supporters to hold their ground, saying he feared an imminent offensive on the protest site occupied by thousands in the heart of Bangkok’s commercial district for more than six weeks.

It was unclear whether the military presence was the start of the crackdown or the beginning of a long process to raise pressure on the protesters and flush out women, children and others from a sprawling encampment where explosives, guns and grenades are thought to be stashed.

ARMOURED VEHICLES

At least two dozen armored personnel carriers approached the encampment.

Slideshow ( 34 images )

“We’re asking everybody to be ready for a crackdown because armored personnel carriers are beginning to move in (to the area),” said Nattawut Saikua, a protest leader.

Some troops in the business district were as close as 200 meters (650 ft) from the protesters’ three-meter (10-foot) high barricade made of tires, bamboo poles, and concrete topped with razor wire, a Reuters photographer said. Pick-up trucks and buses carrying soldiers also arrived in the area.

Slideshow ( 34 images )

About 3,000 of the mostly rural and urban poor protestors, who broadly supporters of former premier Thaksin Shinawatra who was ousted in a 2006 coup, remain in the encampment in Bangkok’s high-end shopping, hotel and diplomatic district.

“We have received reports that they will come in some areas. Negotiations are ongoing. But if they come, we will let it happen and fight on from here,” Nattawut told supporters from a stage at the center of the main protest site.

Soon after he spoke, protest leaders broke into songs and, in a surreal sight, ran comedy skits from the stage.

They accuse the British-born, Oxford-educated Prime Minister Abhisit Vejjajiva of lacking a popular mandate after coming to power in a controversial parliamentary vote in 2008 with tacit backing from the military, and have demanded immediate elections.

Troops have thrown a cordon around the 3 sq-km (1.2 sq-mile) protest site, a “tent city” at the Rachaprasong intersection, paralyzing the heart of Bangkok. Hundreds of women and children have taken refuge in a temple inside the protest area.

Deputy Prime Minister Suthep Thuagsuban has said moving in on the encampment had to be “a last resort.”

The red shirts have been stockpiling food, water, and supplies in their encampment since Thursday when troops began an operation to isolate them, sparking several days of street fighting that has killed 39 people and wounded nearly 300 in Thailand’s deadliest political violence in 18 years.