An anti-government protester throws a rock at Thai riot police.

[Updated at 12:13 p.m.] An anti-government protester was shot and killed Thursday as a tense standoff continued on the streets of Bangkok.

Gunfire rang through the streets as the situation became increasingly volatile.

Earlier, the leader of the Red Shirt movement, Seh Daeng, was shot in the head while being interviewed, according to a journalist who witnessed the shooting. He was in critical condition.

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[Updated at 10:50 a.m.] Video footage taken just after the shooting in Bangkok showed the leader of anti-government protests lying on the ground, dressed in camouflage, as frantic protesters attempted to move him and get help.

Seh Daeng of the Red Shirt protesters appears to be bleeding from a head wound in the video. Footage from the hospital showed medics covering Seh's face as he was brought in on a stretcher amid a throng of media.

Thai soldiers in riot gear patrol streets in Bangkok near the 'Red Shirt' anti-government camp.

[Updated at 9:40 a.m.] A leader of anti-government protests in Thailand was shot in the head while being interviewed at the demonstrations, according to Tom Fuller of the International Herald Tribune told CNN he was interviewing Seh at the time of the shooting.

[Updated at 9:26 a.m.] Witnesses said the gunshot appeared to come from a rooftop in a corner of Bangkok's Lumpini Park, where protesters have amassed.

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.

[Updated at 9:01 a.m.] A leader of anti-government protests in Thailand was in critical condition Thursday after being shot in the chest, according to his guards. Seh Daeng of the Red Shirt protesters was hospitalized, the guards said. Other sources confirmed the shooting.

A renegade Thai general, Seh is one of the Red Shirts' most radical, hard-line leaders, said CNN's Dan Rivers. Other, more moderate leaders of the organization, who had insisted the protests were peaceful, had sought to distance themselves from him.

[Posted at 8:48 a.m.] Explosions and gunfire were heard Thursday near the site of anti-government protests in Bangkok, Thailand, and at least one protest leader was shot, according to guards with the Red Shirt protesters group.

The condition of Red Shirt leader Seh Daeng was unknown.

The Red Shirt guards said that grenades had landed in the protest area.

- CNN's Kocha Olarn contributed to this report.

This story is developing. We'll bring you the latest information as we get it.