Tens of thousands of protesters converged in Bangkok on Sunday 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 in a stream of vehicles from northern provinces over the weekend, carrying red flags and blaring music about democracy and freedom.

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

Investors are worried about violence and the government being distracted when it should be focused on a nascent recovery in Southeast Asia's second-biggest economy.

Protest leaders insist their rally will be peaceful. They 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.

"If Abhisit does not quit by Monday, we will march all over Bangkok," said Veera Musikapong, chairman of the protest group, the United Front for Democracy against Dictatorship (UDD).

Abhisit's government must go to the polls by the end of next year, and is unlikely to agree to immediate elections.

Thailand's security forces were on their highest alert, said Thawil Pliensri, secretary general of the National Security Council, told Reuters.

"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."    

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

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