Thousands of protesters gathered in Bangkok on Sunday and planned to give Thailand's military-backed government an ultimatum: either call elections or face more pro-democracy demonstrations over the coming week.

About 80,000 red-shirted supporters of former premier Thaksin Shinawatra, ousted in a military coup in 2006, arrived on Saturday, many traveling in pick-up trucks, motorcycles or vans from northern provinces, carrying red flags and blaring music about democracy and freedom.

Thousands more were expected on Sunday, including hundreds who boarded boats in nearby Ayuddhutthya province.

Investors are worried about violence, and about the government being distracted when it should be concentrating on nurturing the economy as it recovers from a brief recession.

But Thailand is still benefiting from investment funds flowing into Southeast Asia and foreigners have snapped up $500 million of Thai stocks so far this year, much of it this month. The stock market rose 1 percent on Friday.

The protesters plan to maintain pressure on Prime Minister Abhisit Vejjajiva to dissolve parliament and call an election Thaksin's allies would be well placed to win, or face more protests over a 20 km (12 miles) radius of their main protest site on Ratchadamnoen Road in the heart of Bangkok.

It is expected to get up to over 100,000 people today, Thawil Pliensri, secretary general of the National Security Council, told Reuters. Protest leaders say they already had drawn hundreds of thousands to Bangkok as of Saturday night.

The security forces are on highest alert. The situation remains normal and we expect it to remain so today, he added. But it may get more volatile after a few days as the protest leaders step up their measures and people are tired and frustrated. We have to make sure there is no damage. We are here to ask for justice and for rule of law to be applied to all, one protest leader, Weng Torirajkan, told supporters who responded with loud cheers.

Since the government cannot do it because it's too busy serving the elite, we ask that it step out and call fresh elections so we have a government that represents the whole country, a government that represents us.

About 40,000 soldiers and police have been mobilized.

TARGETING BANGKOK'S ELITE

The protests add a new chapter to a seemingly intractable political conflict pitting the military, the urban elite and royalists, who wear the revered king's traditional color of yellow at protests, against the mainly rural Thaksin supporters.

The protesters say the Oxford-educated Abhisit came to power illegitimately, heading a coalition cobbled together by the military after a pro-Thaksin party leading the previous coalition government was dissolved by the courts.

The red shirts chafe at what they say is an unelected elite preventing allies of twice-elected Thaksin from returning to power through a vote. Adding to their anger, Thailand's top court seized $1.4 billion of his assets last month, saying it was accrued through abuse of power.

On Saturday, a Reuters photographer counted thousands of pickup trucks streaming into Bangkok along a main road from Thaksin's strongholds in the north and northeast of the country. Police briefly stopped them and searched for weapons.

Thousands had gathered on Ratchadamnoen Road, a central Bangkok thoroughfare leading to the bridge that will be the main site for the Sunday rally.

Armed guards stood at many banks and state buildings after government warnings of potential sabotage, including bombings.

In 2008, a rival group sought to topple a Thaksin-allied government by seizing Government House for three months and shutting Bangkok's two main airports for eight days, damaging the tourist sector and dealing a blow to investor confidence.

The UDD insist they will not use the same tactics.

Roberto Herrera-Lim, an analyst at Eurasia Group, said the red shirts recognized violence would be counter-productive and were now intent on building up public support for what they present as a pro-democracy movement.

Whether the 'red shirts' will accomplish anything at all depends on the numbers that they can muster, he said.

Thaksin fled Thailand in 2008 to escape a two-year sentence for graft. Woravat said he was in Dubai, now his main base, but

would be flying to Europe soon to meet up with his daughters.

Government House, which includes Abhisit's office, has been cordoned off. The authorities have closed several other roads to prevent protesters from besieging government buildings.

(Writing by Jason Szep; Editing by Jerry Norton)

Thailand Elections