"Riot police have clashed with protesters and have fired tear gas at them. Shots have also been fired," Al Jazeera's Wayne Hay, reporting from Bangkok, said.
"Red shirts have won this battle and have been able to push the military back - the military have now turned around and gone down the road."
Emergency forces reported that more than 33 people were injured in the clashes as the tensions escalated in the biggest confrontation after weeks of mass rallies.
"The army will not kill Thais but we have to restore law and order," Sansern Kaewkamnerd, the army spokesman, said.
Troops had been ordered to reclaim the Phan Fah bridge and Rajdumnoen Road area, near several government and army buildings in the Thai capital.
There were about 4,000 protesters, including women and children, left in the area and the troops had about 200 metres to clear before they get to the main stage, Sansern said.
Hundreds of troops also massed at a downtown protest site, apparently ready to move in, and the crowd was growing by the hour.
Thai troops earlier used water cannons to repel anti-government protesters from an army base in central Bangkok, local television reported.
The protesters, known as "red shirts", stormed the base near the Phan Fah bridge in the Thai capital on Saturday but did not enter it, local PBS channel has reported.
Al Jazeera's Wayne Hay, reporting from Bangkok, said that tensions were high as riot police gathered near areas where the protesters were camped.
Government forces maintained a heavy presence at the television station they retook control of after claiming that the opposition channel was using it to incite violence.
The move came just hours after protesters had stormed the station and forced the reversal of an earlier decision to ban the channel.
The protesters have warned they will launch serious retaliation against the government but Abhisit Vejjajiva, the prime minister, has vowed not to bend to their demands that he relinquishes power and calls for fresh polls.
"It's not over yet. I'm confident if we stay true to righteousness, we will win the day," Abhisit said in a nationwide television address late on Friday.
But the prime minister gave no indication that he would veer from his so far strict adherence to non-violent measures.
Panitan Wattanagakorn, a spokesman for Abhisit, told Al Jazeera from Bangkok that the station was closed because it was airing misleading information that the government would use force to crush the people.
"We are planning to arrest some of the protest leaders who are clearly breaking the law," he said.
Panitan said the government will avoid conditions that will create more tension and will try to negotiate with protesters and ask them to surrender.
"The prime minister does not rule out negotiation with them but in order to do that they need to respect law and order first."
|Violent clashes have caused injuries among protesters and security forces [Reuters]
Red shirt leaders announced that they would retaliate and vowed to ratchet up pressure on the government.
On Friday, hundreds of security forces, using tear gas and water cannons, attempted to keep the red shirts out of the Thaicom Pcl satellite station building, but after protesters forced their way through a security cordon, troops backed down and officials agreed to restore the channel to air.
Nattawut Saikua, a red shirt leader, told a cheering crowd on Friday night at the commercial district rally site that the routing of the troops at the station was "a major victory".
Nattawut is one of some two dozen red shirt leaders for whom arrest warrants have been issued. So far, none have been taken into custody.
The Thaicom Pcl satellite station, which relays the People's Channel signal via satellite, was founded by Thaksin Shinawatra, the ousted former prime minister that many of the red shirts support, but he no longer owns it.
"This is an explosive situation but both sides know that violence, like what happened in Kyrgyzstan, is not an option," Supong Limtanakool, professor at Bangkok University, told Al Jazeera.
“The side that starts the violence will be condemned. Thaksin has a chance to show that he has the country’s best interests at heart, by reaching a negotiation with the government."
The authorities had taken over the channel on Thursday, triggering some 12,000 protesters to move to the station in the suburb of Pathum Thani, about 50km north of Bangkok, on Friday.
"The station is inciting violence and spreading false information and rumours to the detriment of the country"
Sansern Kaewkamnerd, Thai army spokesman
Sansern Kaewkamnerd, an army spokesman, said security forces stood down "because we did not want any loss of life or to hurt anyone".
But more than 4,000 soldiers retook the satellite transmission complex late on Friday after Abhisit ordered that the People Channel be blocked again.
"The station is inciting violence and spreading false information and rumours to the detriment of the country," Sansern said.
Complaints about Abhisit's leadership have risen since the red shirts, who had established a base last month in the old part of Bangkok, spread to a new encampment a week ago at an intersection in the heart of the capital's tourist and shopping district.
Merchants say the boisterous demonstrations have cost them millions of dollars and luxury hotels near the site have been under virtual siege.
On Wednesday, Abhisit's government declared a state of emergency, allowing it to impose curfews, ban public gatherings, censor media and detain suspects without charge for 30 days.