Bangkok, Thailand (CNN) -- Military troops on Wednesday morning began entering a park in central Bangkok, where thousands of opposition protesters have been camped out in defiance of a government order to vacate the area.

Armored personnel carriers were seen smashing into bamboo and tire barricades lining Lumpini Park, the site of the main demonstration area for the so-called Red Shirt protesters. Soldiers were also seen shooting sporadically as they entered the northwest edge of the park.

The large show of force appeared to be the beginning of a large military operation to root out remaining protesters two days after a government-issued deadline expired with many Red Shirts still holding ground.

"This will be the last operation by the government," Thai senator Lertrat Ratanavanich said on local television. "It is impossible to avoid the loss."

The prime minister's office issued a statement blaming the crackdown on failed talks between the two sides.

"Negotiations failed because core (opposition) leaders are not to be able to make decisions by themselves," the statement said, alluding to an outside force influencing the protesters. "(We) ask core leaders to stop the rally and surrender."

A government statement read on Thai television announced that the operation will continue throughout the day and sought to assure residents that security officers were working to secure their safety.

"The Royal Thai government would like to inform the residents of Bangkok that today the security officers will operate in several areas to secure a perimeter," the statement said. "We are going to make sure that within the perimeter security and safety will be provided to the public."

Earlier Wednesday a CNN correspondent positioned on a building overlooking the park said it appeared that most of the protesters had dispersed by the time the troops entered around 8:30 a.m. Wednesday (9:30 p.m. Tuesday ET). The park had been housing as many as 5,000 protesters just a day earlier.

At least four people were injured in the initial crackdown, medical officials said.

Several hundred troops began amassing just as dawn broke over the city. It was the largest movement of forces since clashes broke out last Thursday between opposition protesters and government security officials.

The troops, many of whom were armed, were seen walking in a long column and carrying razor wire and fire extinguishers near the park where the protesters have been launching demonstrations.

Armored personnel carriers also were spotted near the protest site, and gunfire could be heard nearby.

A large plume of black smoke billowed into the sky from one of three large tire fires the opposition was using as shields. A bank building was also reported to be on fire on the main road where protesters and security forces have traded gunfire for the past six days.

Meanwhile at the main staging area in the middle of the park, opposition operations apparently continued as normal with speeches going on, music being played and leaders telling protesters they can stay as they please, even while saying they want to continue negotiations with the government.

Local television reports said protesters were being told to seek shelter at a nearby temple if they were concerned about the growing troop presence.

At least 36 people have been killed since clashes intensified Thursday.

The violence prompted the United Nations' top human rights official to implore anti-government protesters and government officials to resume talks.

Satit Wongnongtoey, the Thai prime minister's office spokesman, said negotiations can be held when the opposition, the United Front for Democracy Against Dictatorship, ends its protest.

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"I can confirm that the government has always wanted to talk, but it has been let down by the UDD, due to the intervention of a mastermind abroad," said Satit, who didn't identify the person.

The opposition members, also known as Red Shirts, support former Prime Minister Thaksin Shinawatra, who was ousted in a 2006 bloodless military coup.

The capital was notably calmer Tuesday. But after more than five days of violent standoffs, debris and piles of tires littered battle-scarred streets, and the sound of gunfire still regularly punctuated the air.

Police spokesman Col. Songphol Wattanachai told reporters Tuesday that police had seized 9,021 tires from the city's streets. Burning tires have been used by protesters to create shields of black smoke during recent clashes.

Songphol said police had arrested and were interrogating a Red Shirt protester who was a close aide to Maj. Gen. Khattiya Sawatdiphol, better known as Seh Daeng -- a renegade leader of a violent anti-government faction who died this week after being shot in the head by a sniper.

As troops continued their crackdown on protesters, Amnesty International criticized the government's approach.

Timeline of Thailand's political crisis

Benjamin Zawacki, the organization's Thailand specialist, told CNN that 35 of the people killed since Thursday were unarmed, including a 17-year-old boy and two medics.

"Our concern is that the government is using live ammunition or live rounds preemptively, rather than as a last resort, and using them against persons who are unarmed and present no credible threat to the soldiers or anyone else," he said.

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But government officials maintained that they were following rules of engagement. Troops only use live bullets when first attacked by terrorists with war weapons, Col. Sansern Kaewkamnerd told reporters.

Two main groups of anti-government demonstrators have been demanding that Prime Minister Abhisit Vejjajiva dissolve the lower house of Parliament and call new elections: the Red Shirts, whose leaders claim that protesters are peaceful, and the Black Shirts, who advocate a more violent approach.

What are the protests about?

The government ordered all demonstrators to leave their protest site by 3 p.m. Monday, but thousands continued to hold their ground.

"As the latest government deadline passes, there is a high risk that the situation could spiral out of control," UN High Commissioner for Human Rights Navi Pillay said Monday.

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"To prevent further loss of life, I appeal to the protesters to step back from the brink, and the security forces to exercise maximum restraint in line with the instructions given by the government. Ultimately, this situation can only be resolved by negotiation."

The Ministry of Public Health reported that 65 people have died and more than 1,000 have been wounded since the Red Shirt anti-government protesters began flooding the streets of Bangkok on March 12 to demand new elections.

CNN's Kocha Olarn contributed to this report.