BANGKOK: Leaders of tens of thousands of protesters who swarmed into Bangkok from Thailand's rural areas Sunday threatened mass street demonstrations if the government didn't respond to their demand for a dissolution of Parliament within 24 hours.

The demonstrators, popularly known as the Red Shirts, want Prime Minister Abhisit Vejjajiva to call new elections, which they believe will allow their political allies to regain power. They believe Abhisit came to power illegitimately with the connivance of the military and other parts of the traditional Thai ruling class who were jealous and fearful of former Prime Minister Thaksin Shinawatra's popularity while in office from 2001 until he was ousted in a 2006 coup.

"We're demanding the government give up the administrative power by dissolving the Parliament and returning power to the people," a protest leader, Veera Musikapong, told a sea of red-shirted followers. "We're giving the government 24 hours from now (to respond to our demand)."

The protesters, who had Saturday given a similar ultimatum which expired Sunday, said they will march on key locations in the city if the government fails to respond by noon Monday (0500 GMT; midnight EST). These are to include the headquarters of the 11th Infantry Regiment, where Abhisit has been living in recent days.

In his weekly radio address Sunday morning, Abhisit indicated he had no plans to dissolve Parliament.

"Dissolution and call for resignations are normal in a democratic system. But we have to make sure the dissolution of Parliament will solve the problem and won't make the next election troublesome," Abhisit said.

He also denied rumors that a military coup was possible and said he would not impose a state of emergency that would give the army broad powers to deal with the protests.

Traffic in Bangkok was light, businesses were shuttered and social events canceled as many feared the four-day demonstrations, which officially began Sunday but have been building for two days as caravans of protesters pour in from the north and northeast, would repeat past violence. But protesters stressed they would use only peaceful means in their quest for new elections.

One protest leader, Jatuporn Prompan, described it as "the biggest war of the common people in the country's history," and said he expected a million people to gather by noon Sunday. But police Gen. Wichai Sangprapai, commander in the main protest area, estimated that the number of protesters throughout Bangkok could reach 150,000 Sunday.

Local newspapers estimated the numbers at between 80,000 and 100,000, although more were still arriving from outlying areas, traveling in trucks, buses, motorcycles and boats down the Chao Phraya River.

A force of 50,000 soldiers, police and other security personnel was mobilized in the capital area.

There were no reports of violence, and both Abhisit and Jatuporn praised authorities for facilitating the protesters' easy entry into the capital. Abhisit said the government has asked protest leaders to monitor any groups among the demonstrators who may want to provoke violence.

Despite newspaper headlines warning of a "red tide" about to swamp the city, the protests took on an almost festive atmosphere with musical performances and dancing interspersed with political speeches.

The march is regarded by some as the last chance for Thaksin to return to Thailand.

The protesters, formally known as the United Front for Democracy Against Dictatorship, are made up of followers of Thaksin, along with other people who oppose the coup that toppled him.

Forcing the government out of power, Thaksin loyalists say, could pave the way for his pardon and return.

Thaksin, who resides in Dubai, United Arab Emirates, faces a two-year prison term for abuse of power. But he remains especially popular among the rural and urban poor who are thankful for the cheap medical care, low interest loans and other measures his government enacted to alleviate poverty.

On Saturday night, Thaksin telephoned the protesters' People TV station to deny rumors that he had been expelled from the United Arab Emirates and was in neighboring Cambodia. Thaksin said he was on a visit to Europe.

Thailand has been in constant political turmoil since early 2006, when demonstrations accusing Thaksin of corruption and abuse of power began. In 2008, when Thaksin's political allies came back to power for a year, his opponents occupied the prime minister's office compound for three months and seized Bangkok's two airports for a week.

Recent polls in Bangkok indicate a large segment of the population, irrespective of their political beliefs, is fed up with the protests, which have battered the economy, including the lucrative tourism industry.

The Red Shirts' last major protest in Bangkok in April deteriorated into rioting that left two people dead, more than 120 people injured and buses burned on major thoroughfares. The army was called in to quash the unrest.

Many embassies have warned their citizens to stay away from areas of the city where violence could erupt.

"This government has no intention to crack down on the protesters because that doesn't benefit anyone," Abhisit said, adding the government would strictly follow legal procedures if forced to disperse lawbreakers.