Troops fired repeatedly into an intersection leading to an encampment in a ritzy hotel and shopping district they have occupied for five weeks, a Reuters witness said, adding he saw several people injured including two journalists.
It was unclear if troops were using live rounds, rubber bullets or both, he said.
A Bangkok-based foreign journalist working for France 24 television station was wounded, the station said. A Thai photographer was also shot, a Reuters witness said.
Analysts said a possible split between a police force that has loyalties to ousted premier Thaksin Shinawatra and the military will make it more difficult to contain the violence.
Those fears were underlined after a Thai policeman fired bullets at soldiers during the clashes, a Reuters witness said.
Troops had yet to fully seal off all major roads leading to the "red shirt" encampment, raising questions over whether the government could soon end the protests.
At least one person was killed 11 people wounded in the violence since Thursday night, but that toll was expected to rise. Many hospitals declined to provide numbers of casualties.
The crisis, in which 30 people have been killed and more than 1,400 wounded since April, has paralyzed parts of Bangkok, scared off investors and has begun to hit the wider economy.
The cost of insuring Thai debt jumped the most in 15 months and Thai bond yields fell to a nine-month low on Friday as the wave of violence prompted investors to rush to the relative safety of government debt.
Five-year credit default swaps, used to hedge against debt default but also to speculate on country risk, jumped by more than 30 basis points to 142 basis points.
Stocks fell 1.2 percent.
Protesters had formed their own checkpoint overnight at the famous Suan Lum night market to stop soldiers from sealing off roads around their main fortified encampment in Bangkok's commercial heart. That became one of the main battlegrounds.
They set fire to a bus, motorbike and tires as they retreated, and soldiers took control of an intersection leading to a road lined with hotels, the U.S. ambassador's home and several embassies, which were closed and evacuated.
Troops fired rubber bullets into a nearby park after gunshots were heard, Thai television said.
Soldiers used tear gas and water cannon before dawn at the Nana intersection, packed with shops and racy go-go bars. Skirmishes flared in other parts of the city as the protesters remained defiant, vowing to fight to the death.
"They are tightening a noose on us but we will fight to the end, brothers and sisters," a protest leader, Nattawut Saikua, told a cheering crowd of about 10,000 at the main protest site.
SEALING OFF PROTEST SITE
The latest violence followed tough security measures imposed on Thursday evening to reclaim Bangkok's commercial district after the collapse of a reconciliation plan proposed last week by Prime Minister Abhisit Vejjajiva.
Abhisit is under enormous pressure to end the protests, which began with festive rallies on March 12 and descended into violence that is stoking concerns over the outlook of Southeast Asia's second-biggest economy.
The shooting and a security cordon marked the start of a violent crackdown in which the Thai government stands a good chance of clearing the streets, the Eurasia Group political risk consultancy said.
"But it will not end the polarization that has led to the current instability -- ensuring that the pressure from the red shirts will persist and that political volatility will remain a persistent problem for Thailand for the foreseeable future".
It is unclear who shot a renegade general who has been in charge of security for thousands of protesters occupying a 3 sq-km (1.2 sq-mile) stretch of central Bangkok since April 3.
Khattiya Sawasdipol, a suspended army specialist better known as "Seh Daeng" (Commander Red), was shot in the head, apparently by a sniper, while talking to reporters on Thursday evening.
He underwent brain surgery and was in critical condition.
Khattiya had been branded a terrorist by the Thai government, which accused him of involvement in dozens of grenade attacks that have wounded more than 100 people.
But in recent days he was equally critical of other red shirt leaders, accusing them of embracing Abhisit's proposed "national reconciliation" which unraveled after protesters refused to leave the streets.
Speculation was rife as to who might have tried to assassinate him with fingers pointing at the military, shadowy militants who have appeared in previous incidents of violence, and from the ranks of red shirts themselves.
His shooting sparked half a dozen confrontations overnight between rock-throwing protesters and armed security forces on the outskirts of the protesters' barricaded encampment.
One protester was shot in the eye and died after a group of red shirts confronted soldiers armed with assault rifles next to a park in the Silom business district, witnesses said. Some protesters hurled rocks and troops fired in return.
Most businesses and embassies in the area have evacuated staff and were closed for the day. Apartment complexes were mostly empty after the government warned it would shut down power and water supplies, and landlords urged tenants to leave.Ninjette
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