Several hundred military troops began massing in central Bangkok early Wednesday - the largest movement of forces since clashes broke out last Thursday between opposition protesters and government security officials.
The troops were seen walking in a long column and carrying razor wire and fire extinguishers near the site where the protesters have been launching demonstrations.
Opposition leaders told CNN in a phone call that armored personnel carriers were also spotted near the protest site, but that report could not be immediately confirmed.
It was not clear where the troops were heading, but the large show of force raised speculation that some sort of military operation could be unfolding to root out about 5,000 protesters still occupying the demonstration area.
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 Wongnongtaey, 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.
"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 city 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 Watanachai 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 Sawasdipol, 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.
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 pre-emptively, 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.
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.
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. "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.