Bangkok, Thailand (CNN) -- Thailand's army had nothing to do with the shooting of an opposition leader in downtown Bangkok, a military spokesman said Friday.
The Thursday incident is under investigation, said Col. Sansern Kaewkumnerd, an Army spokesman.
Violence erupted during anti-government protests in Thailand on Thursday as one demonstrator was killed and the key protest leader was shot in the head while being interviewed by journalists.
Tension soared amid the sound of explosions and gunfire and an anarchic mob atmosphere in downtown Bangkok's Lumpini Park, where protesters had massed.
Video footage showed one protester dead on the pavement. There was also footage of Red Shirt movement leader Maj. Gen. Khattiya Sawasdipol lying on the ground, dressed in camouflage, as frantic protesters attempted to move him and get help.
Tom Fuller of the International Herald Tribune told CNN he was among those interviewing the renegade general -- better known as Seh Daeng, or Red Commander -- when he was shot.
One of the more radical leaders in Thailand's wide-ranging protest movement, Khattiya appeared to be bleeding from a head wound. Footage from the hospital showed medics covering his face as he was brought in on a stretcher amid a throng of media.
He was in critical condition, his guards said.
While it was unknown whether Thailand's military or government was behind the shooting, the government has previously made it clear it would shoot at what it called armed terrorists. There was no immediate claim of responsibility.
A policeman who saw Khattiya's wounds told CNN that he might have been shot by a sniper.
Khattiya's shooting increased volatility on the streets. More gunfire rang out afterward, according to witnesses. It was not immediately clear whether they stemmed from Red Shirt retaliation.
The Thai news agency MCOT said police reported 20 people were wounded Thursday.
The violence erupted after Thai authorities set a new deadline to seal off the Bangkok intersection where protesters have gathered by the thousands for the past month. Officials had said soldiers would seal off roads and shut down rail service leading to the Ratchaprasong intersection at 6 p.m. (7 a.m. ET) Thursday.
However, the road on the park's eastern border remained open as of about 8 p.m. A few soldiers were present.
The United Front for Democracy Against Dictatorship (UDD), the formal name of the Red Shirts, greeted the government's announcement with defiance.
"We want peace but they want war," said Weng Tojirakarn, a party leader. "We will fight with our bare hands. We will stay."
Authorities had initially threatened to shut off power, cut supplies and seal off the intersection at midnight Thursday. They postponed the plan because they wanted to limit the impact on area residents, said Panitan Wattanayakorn, the acting government spokesman.
The Centre for the Resolution of the Emergency Situation in Bangkok said it has now asked businesses in the area to shut down until the situation is resolved.
The government said it has been forced to take action after demonstrators disregarded an ultimatum by Prime Minister Abhisit Vejjajiva to vacate the intersection by Wednesday.
The UDD has turned the posh commercial center into a fortress of tires and bamboo sticks as they continued to demand that Abhisit dissolve the lower house of Parliament and call new elections.
The Red Shirts support former Prime Minister Thaksin Shinawatra, who was ousted in a bloodless military coup in 2006.
Spokesman Panitan said continuing protests by Red Shirts would affect the proposed election date of November 14. He said Abhisit had mentioned that the proposed date for the new election had been made with the condition that the Red Shirts stop their protests. "He never said he withdrew his proposal," Panitan said.
More than two dozen civilians and military personnel have died in police-protester clashes in the ongoing unrest.
Over the weekend, two Thai police officers were killed and eight people wounded in violence that began Friday night and lasted into early Saturday.
CNN's Dan Rivers and Kocha Olarn contributed to this report.