Thousands of anti-government protesters in Thailand have begun rallying outside on a military base on the outskirts of Bangkok as they step up pressure on the country's prime minister to stand down and call fresh elections.

The protesters, known as the reds shirts, have set a deadline of noon on Monday for Abhisit Vejjajiva to resign or face what they say will be crippling mass demonstrations in the capital.

Red shirt leaders have warned of further disruption if thePM does not stand down

Amid the escalating protests Abhisit has ordered tens of thousands of soldiers and police on to the streets of Bangkok and moved his government to the 11th Infantry Regiment headquarters outside of Bangkok.

But he has indicated he has no plans to dissolve parliament.

"We will march over there, brothers and sisters. We will go to the infantry to get an answer from Abhisit himself," said Nattawut Saikua, a leader of the red shirt protesters known formally as the United Front for Democracy against Dictatorship (UDD).

"With this many people on the streets, I don't see how he still thinks he has any legitimacy."

Thousands of red shirt demonstrators began gathering in Bangkok on Friday, with around 150,000 attending a huge rally in the city on Sunday.

Most red shirts are supporters of Thaksin Shinawatra, the former Thai prime minister ousted in a 2006 coup.

They believe the current government, led by Abhisit, did not take power legitimately and is propped up by segments of Thailand's traditional ruling class who were threatened by Thaksin's popularity, especially among the poor.

Many of the protesters travelled to Bangkok from Thailand's poor, rural northern and eastern provinces – areas where support for Thaksin has traditionally been strongest.

So far the protests have been peaceful and UDD leaders have pledged they will remain that way.

But Monday's threat to step up disruption if Abhisit resists calls to stand down could spark anger by paralysing traffic-choked streets all over Bangkok.

Abhisit himself has warned the public not to be complacent about the potential for violence.

In his weekly television address on Sunday, the Thai prime minister indicated immediate elections were unlikely, citing the tense political climate and his government's parliamentary majority.

Several main roads near government offices were blocked off either by protesters' pick-up trucks and motorcycles, or cordoned off by police and soldiers.

Authorities have deployed 50,000 police, soldiers and other security personnel across the city, with thousands more placed on alert at barracks across the city.

An army spokesman said security had been beefed up with hundreds of extra troops at the infantry base where Abhisit and other key ministers along with the country's top brass were staying during the protests.

Suthep Thaugsuban, the Thai deputy prime minister in charge of the country's security, said the protesters would be permitted to approach the base but added that it was too early to say if emergency rule would be invoked to crackdown on the rally.land's Red ShirtsThailand's Red Shirts

"Protesters can circle the barracks but they must not obstruct traffic or intrude into the barracks or they will be arrested," Suthep told reporters.

"We have to see how the situation develops but I reaffirm that we will act reasonably and appropriately."

Last April, mass protests by the red shirts led to violent clashes on the streets of Bangkok with at least two people killed and 120 injured in the capital's worst unrest in almost two decades.

Red shirt leaders say the violence was stirred up by gangs of hired pro-government thugs.

The Red Shirts are loyal to Thaksin, who was ousted in a coup in 2006 and is living abroad, mostly in Dubai, to escape a jail term for corruption.

Last month Thailand's supreme court confiscated $1.4bn of his assets after ruling the money had been obtained through abuse of power when he was prime minister.

In an address to supporters on Sunday night he urged them to keep up the fight against those in the Thai elite he says are destroying the country's democracy.

"This is not a one-person issue, we all fight for justice. I am the victim of bullies among the elites," Thaksin told the rally in a 50-minute video message.

"To solve problems related to democracy, equality and justice, the ruling elites won't be able to do that because they don't have the conscience. The people will have to do it."

Thailand has been in constant political turmoil since early 2006, when anti-Thaksin demonstrations began following his ouster in a military coup.

The Red Shirts have held a number of rallies since Abhisit came to power in December 2008, after a court decision removed Thaksin's allies from government.