Thai Prime Minister Samak Sundaravej said on Saturday he would crack down on anti-government protests that have ignited fears of a military coup.

"I will not yield to you," Samak said on national television a day after the resignation of a cabinet minister that was meant to head off street protests similar to the campaign against Prime Minister Thaksin Shinawatra before a 2006 coup.

He threatened police and military action against protesters if they did not leave the Makawan Rangsan Bridge near the gilded Grand Palace in the heart of Bangkok.

The street protests began last Sunday when 5,000 opponents of the coalition government which backs Thaksin held a rally in the capital.

The prime minister has accused the anti-Thaksin People's Alliance for Democracy (PAD), which vowed on Friday to step up protests against the government, of damaging the country.

"You have broken the law. I have a duty to deal with you. You cannot stay there. I will take you out," said Samak, who leads a pro-Thaksin coalition government elected last December.

At the rally, speakers shrugged off Samak's threat and vowed to stay put as the crowd swelled to around 5,000 from just over 1,000 earlier on Saturday.

Deputy Police spokesman Surapol Thuanthong said protest leaders would be ordered to end the rally to ease traffic congestion and there would be negotiations for them to move away peacefully.

Several hundred riot police with shields and batons took up position at barricades later on Saturday, although the mood in the crowd appeared festive.

"What we know for sure is that if any violence occurs, it will come only from the government side, not ours," Somsak Kosaisuk, one of five PAD leaders, told Reuters.

Minor scuffles broke out between pro- and anti-Thaksin protesters last Sunday, stoking fears that the army might seize the chance of social unrest to storm back into the political fray, analysts said.

CROWD TOLD NOT TO RESIST POLICE

Somkiet Pongpaiboon, another PAD leader, told the crowd not to resist if police tried to end the protest, which has shut a 1 km stretch of a six-lane city centre road for nearly a week.

Thailand's top military commander, who denied reports on Thursday that the army may be plotting another coup, said he did not believe the army would be called into the streets of Bangkok.

"That would require a state of emergency and I don't think the Prime Minister will do that. It would make the country look bad," Supreme Commander Boonsrang Niumpradit told Reuters.

Jakrapob Penkair, Minister to the Prime Minister's Office, quit on Friday after he was accused by police of making offensive remarks against the revered King Bhumibol Adulyadej in 2007.

The saga is part of wider campaign by the PAD and opposition Democrat Party to paint Thaksin and his acolytes as republicans who want to end the country's 76-year-old constitutional monarchy.

Stock investors reacted positively to Jakrapob's resignation, with the main index closing up 0.4 percent on Friday after four days of falls due to political uncertainty.

Analysts say any violence could prompt the military to intervene, unsettling foreign investor confidence in an economy struggling with slowing growth and soaring inflation.

Samak, who also holds the post of defence minister, said the police and army were fully behind him.

"This is not September 19," he said, referring to the date of the bloodless 2006 coup. "We know what the problem is and we will deal with it. The situation does not warrant another coup."