Thai authorities were today set to step up security measures in the capital after fierce clashes between troops and "Red Shirt" anti-government protestors left 10 people dead and 125 injured.

Troops opened fire on protesters Friday after a military lockdown of their vast fortified rally site in the heart of Bangkok sparked fierce clashes.

Soldiers have blocked roads and set up checkpoints to seal off the area around the wider protest area, which extends for several square kilometres.

The protesters, who are trying to bring down the government of Prime Minister Abhisit Vejjajiva, threw stones, used slingshots and launched fireworks at the troops as the two-month standoff descended into more violence.

Nine men and one woman, all civilians, were killed, according to the official Erawan emergency centre.

Three journalists, one a Canadian with the France 24 television channel, were shot and wounded covering the unrest, underscoring the risks facing media in the capital, where a Japanese cameraman was killed last month.

Government spokesman Panitan Wattanayagorn said troops had come under attack as they moved to seal off the rally area to prevent more Red Shirts from entering.

"The soldiers... had no choice but to respond to these attacks," Panitan told a news conference Friday. Troops were authorised to use live ammunition in self-defence, for warning shots or against armed gunmen, he added.

Troops would step up security measures in the coming days to search for weapons and reduce the number of people entering the area, he said, warning of the risk of more instability in the capital.

The United States urged all those involved in the political crisis to show restraint and resolve their differences peacefully.

"We are deeply concerned about the situation on the ground in Thailand," State Department spokesman Philip Crowley told reporters.

"We continue to strongly encourage everyone involved... to show restraint and to find a way to work peacefully through these differences and do so in a way that strengthens democratic institutions," Crowley added.

Early polls plan shelved

The violence came after Abhisit shelved a plan to hold early elections in November after reconciliation efforts broke down.

The mostly poor and working class Reds consider Abhisit's government illegitimate because it came to power in a 2008 parliamentary vote after a court ruling ousted elected allies of their hero, former premier Thaksin Shinawatra, who was unseated in a 2006 coup.

Thaksin, who lives in self-imposed exile to avoid a jail term for corruption, called on the government to pull back troops and restart negotiations with the demonstrators.

"The government's actions clearly constitute grave infringement of human rights and criminal offences," Thaksin said in a statement.

Defence Minister General Prawit Wongsuwon said the military operation was meant to force the movement's leaders back to talks with the government.

At least 40 people have been killed and about 1,100 injured in Bangkok in a series of confrontations and attacks since the protests began in mid-March.

Almost one-third of Thailand, including Bangkok, is now under emergency rule.