BANGKOK Thai troops fired rubber bullets and tear gas at thousands of demonstrators, who fought back with guns, grenades and petrol bombs in riots that killed 12 people, Bangkok's worst political violence in 18 years.
At least 521 people, including 64 soldiers and police, were wounded in the fighting near the Phan Fah bridge and Rajdumnoen Road in Bangkok's old quarter, a protest base near government buildings and the regional U.N. headquarters.
Twelve people died, including three soldiers, an emergency medical center said.
Among those killed was Reuters TV cameraman Hiro Muramoto, a 43-year-old Japanese national who had worked for Thomson Reuters in Tokyo for more than 15 years and had arrived in Bangkok on Thursday to cover the protests.
"I am dreadfully saddened to have lost our colleague Hiro Muramoto in the Bangkok clashes," said David Schlesinger, Reuters Editor-in-Chief.
"Journalism can be a terribly dangerous profession as those who try to tell the world the story thrust themselves in the center of the action. The entire Reuters family will mourn this tragedy."
Hundreds of "red shirt" protesters also forced their way into government offices in two northern cities, raising the risk of a larger uprising against Prime Minister Abhisit Vejjajiva and his 16-month-old, military-backed government.
Washington urged both sides in the conflict to show restraint.
"We deplore this outbreak of political violence in Thailand, our long-term friend and ally, and urge good faith negotiations by the parties to resolve outstanding issues through peaceful means," White House spokesman Mike Hammer said.
Tension and uncertainty gripped the city of 15 million overnight, with protesters still on the streets and no sign of the government meeting demands for Abhisit to leave the country and an immediate dissolution of parliament.
In a televised statement, Abhisit expressed regret to the families of the victims and said the army was only allowed to use live bullets "firing into the air and in self-defense".
After hours of violence, army spokesman Sansern Kaewkamnerd said troops would pull back in the old quarter as the riot spread into Khao San Road, an area popular with back-packing tourists.
Khao San Road resembled a war zone, a Reuters photographer said. Shop windows were shattered. Cars were smashed. Many people lay wounded on the street. Police told reporters some protesters had ignited cooking gas cylinders and rolled them into troops.
"If this continues, if the army responds to the red shirts, violence will expand," Sansern said after announcing troops were withdrawing from the area.
He said soldiers had been pelted with petrol bombs and M79 grenades, and that some of the protesters were armed with guns.
A red shirt leader later called on supporters to pull back to the main protest sites.
BARRICADED SHOPPING DISTRICT
Troops mounted two major offensives on protesters in the Phan Fah bridge and Rajdumnoen Road area. Both times they fired rubber bullets and tear gas but failed to clear the area.
An afternoon offensive ended in a standoff with many wounded. After dark, troops opened fire again with rubber bullets about 500 meters (1,600 feet) away at an intersection leading to Khao San Road. Some fired live rounds. Helicopters dropped tear gas.
Tens of thousands also remained in Bangkok's main shopping district, a stretch of upscale department stores and five-star hotels held for a week by the mostly rural and working-class red shirts who say they have been marginalized in a country with one of Asia's widest disparities between rich and poor.
The red shirts used taxis and pick-up trucks to barricade themselves in that area, and expanded their control to include several more blocks. Hundreds of riot police who massed at one end retreated after being surrounding by red shirts.
The violence comes exactly a year after about 10,000 of the supporters of former Prime Minister Thaksin Shinawatra brought traffic in Bangkok to a standstill for several days, occupying major intersections.
In those protests, red shirts hijacked petrol tankers, torched dozens of public buses and hurled petrol bombs at troops until the army imposed order. Two people were killed and 123 wounded. The latest protests, however, involved more than five times as many protesters spread across several areas of the city.
The protesters say Abhisit lacks a popular mandate after coming to power in a 2008 parliamentary vote following a court ruling that dissolved a pro-Thaksin ruling party. They want immediate elections, which Thaksin's allies would be well placed to win.
The red shirts have won new support from Bangkok's urban poor but have angered middle classes, many of whom regard them as misguided slaves to Thaksin, a wily one-time telecoms tycoon who fled into exile to avoid a jail term for graft.
The government declared a state of emergency in Bangkok last Wednesday to control the protests after red shirts broke into the grounds of parliament, forcing some officials -- including the deputy prime minister -- to flee by helicopter.
(Additional reporting by Damir Sagolj and Vithoon Amorn; Editing by Nick Macfie)