Eighteen people were killed and more than 800 wounded in Bangkok's worst political violence in 18 years between troops and 'red shirt' protesters demanding that the government step down and call an early election.

The death toll given on Sunday by Erawan Medical Centre rose during the night although the fighting, some of it in well-known tourist areas, had ended after the security forces pulled back late on Saturday and urged the 'red shirts' to do the same.

The city of 15 million appeared calm early on Sunday after what the Nation daily newspaper headlined 'our darkest hour'.

'Yesterday's bloodbath is a wake-up call to halt the slide towards anarchy,' it said in a front-page commentary.

Thai troops had fired rubber bullets and tear gas at thousands of demonstrators, who fought back with guns, grenades and petrol bombs near the Phan Fah bridge and Rajdumnoen Road in Bangkok's old quarter, a base for the month-old protest.

Four soldiers were among those killed. Hundreds of 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 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.

The Thai government said it had appointed a senior prime ministerial aide to make contact with 'red shirt' leaders to try to find a way to halt the confrontations.

The 'red shirts' - supporters of former Prime Minister Thaksin Shinawatra who was ousted in a coup in 2006 - have demanded that Abhisit dissolve parliament and leave the country.

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-defence'.

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 centre of the action. The entire Reuters family will mourn this tragedy.'

After hours of violence on Saturday, army spokesman Sansern Kaewkamnerd said troops would pull back in the old quarter as the riot spread to 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 smashed and many people lay wounded on the street.

'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.

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 metres (1,600 feet) away at an intersection leading to Khao San Road. Some fired live rounds. Helicopters dropped tear gas.

Tens of thousands had 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 marginalised 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. The violence comes a year after about 10,000 pro-Thaksin supporters rioted in Bangkok. The latest protests 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. Thaksin's allies would be well-placed to win fresh elections.

 The 'red shirts' have shown they have support among Bangkok's poor but have angered the middle classes, many of whom regard them as misguided slaves to Thaksin, a former telecoms tycoon who fled into exile to avoid a jail term for corruption.

The government declared a state of emergency in Bangkok on April 7 after red shirts broke into the grounds of parliament, forcing some officials to flee by helicopter.-Reuters