The head of Thailand's armed forces sought Thursday to calm fears of a fresh coup as protests demanding the overthrow of the new government sparked jitters.

Just five months after elections in Thailand brought an end to more than a year of military rule, anti-government rallies are filling the streets, raising fears of political instability and sending the stock market falling.

Supreme Commander General Boonsrang Niumpradit, who has a largely ceremonial role, sought to reassure people that the army would only leave their barracks to enforce law and order if the current protests escalate.

"A coup is not a clever thing and I don't see any potential coup leader, but if the government asks, troops must go out to maintain order and minimize damage," Boonsrang told reporters.

The People's Alliance for Democracy (PAD) has so far gathered crowds of up to 8,000, but the group is attempting to mount protests similar to its campaign in early 2006, which eventually helped lead to the toppling of the government.

The protesters have since Sunday barricaded a section of a major avenue in the historic district, demanding that Prime Minister Samak Sundaravej stop trying to amend the country's new constitution.

The current charter was drafted by a panel named by royalist generals who staged the 2006 coup against former premier Thaksin Shinawatra.

Samak's drive to amend the charter has proved deeply divisive among Thailand's political elite, provoking rumours that the military could stage a new coup to prevent any changes.

Samak had originally proposed amending only a few key clauses, notably the articles allowing courts to dissolve political parties over election fraud.

But his People Power Party (PPP) now says it would rewrite almost everything except the preamble.

The PAD is calling for a large rally Friday evening, but it is unclear whether it will attract enough protesters to achieve its stated aim of toppling the government.

"If it's less than 10,000 then it has a nagging significance. If it's a high five digits or six digits then you're talking about a critical mass," said Thitinan Pongsudhirak, head of international security studies at Chulalongkorn University.

"It's a combustible situation that puts pressure on this crisis. The PAD is ratcheting up pressure. We may be looking at weeks before an untenable situation arises," he added.

One charismatic minister from the PPP, which is allied to Thaksin and won December elections, is also being accused of disloyalty to the monarchy, one of the reasons that the army gave for ousting Thaksin.

Jakrapob Penkair is facing calls for his resignation and possible legal charges of insulting the royal family.

Thailand's finance minister on Wednesday sought to reassure investors and consumers after stocks fell for three straight days.

"We are confident that everything will proceed within the democratic boundary," said Finance Minister Surapong Suebwonglee.

Despite the similarities to early 2006, political analyst Giles Ji Ungpakorn told AFP that he did not think the government was in immediate danger.

"The middle class that is the base of the PAD is very fickle and probably does not want to see a mass disruption," said Giles, author of a book about the 2006 coup.

"The protests are much much smaller than before. The longer they go on, they get even smaller. People have to go off and earn a living."

But in Bangkok's historic district where protesters have been camping out, 47-year-old hairdresser Kanyarat Chudaeng and son Wiwat, 25, said they were willing to bear a few hardships to get their point across.

"It is not fun to be here, (it is) extremely hot during the day, but we cannot let down our country to be abused by this nominee government and bad politicians," Kanyarat said.

"How can I let the PAD fight alone when they have tried hard to save our country from bad politicians?" said Paphada, a 57-year-old vendor.