BANGKOK: Tens of thousands of supporters of deposed premier Thaksin Shinawatra streamed through military checkpoints into the capital Saturday as they headed to a rally aimed at toppling the government.

Thai authorities have deployed a 50,000-strong security force, including soldiers, to patrol the streets and search protesters as they entered the city, fearing some could incite trouble ahead of Sunday's main rally.

Some 35,000 protesters in their signature red shirts passed through the busiest checkpoint in Ayutthaya, 80 kilometres (50 miles) north of Bangkok, the provincial governor told AFP.

Governor Withaya Pewpong said the Red Shirts, waving red flags and travelling mostly by pick-up truck and car, had been searched by 700 unarmed soldiers and police without incident.

One Red Shirt leader, Kwanchai Praipana, said most protesters from the rural northeast had already reached Bangkok.

"All of us Red Shirts... have already arrived," he told AFP. "We've prepared for a long fight this time but if the government dissolves the house today or Sunday then we are ready to disperse and go home," he added.

Organisers insist the protests will be peaceful, but the government has enacted the strict Internal Security Act to monitor the rally, allowing authorities to set up checkpoints, impose curfews and limit movements.

Thai Prime Minister Abhisit Vejjajiva, who has refused to bow to the protesters' demand to step down and call elections, spoke to reporters after meeting with ministers and top brass at a military barracks on Saturday.

"We should not be complacent because there are some groups of people still wanting to create violence and cause confrontations," said Abhisit, who has cancelled a weekend trip to Australia because of the rally.

Metropolitan police said protesters trickling into the rally site near government ministry buildings had partially blocked traffic and set up giant projectors and mobile toilets next to a large stage.

The government has lowered its estimate of expected turnout at the rally to 70,000, but the Red Shirts say the figure will be nearer 600,000.

The protests come two weeks after Thailand's top court confiscated 1.4 billion dollars of Thaksin's assets, and are the latest chapter in a political crisis that has beset Thailand since Thaksin was toppled in a 2006 coup.

Thaksin, who has been living mostly in Dubai to escape a two-year jail term for corruption at home, has been encouraging his supporters using text messages and his Twitter page.

"Thank you for your dedication.... I want to give my support to the people in the north," he told his followers on Twitter Saturday, before announcing that he was about to fly from Dubai to Europe to see his two daughters.

The protest is set to be the biggest since the Red Shirts rioted in Bangkok in April last year, leaving two dead and scores injured.

The Red Shirts mainly represent Thailand's rural poor, who benefited from Thaksin's populist policies and say Abhisit's government is elitist, military-backed and has ignored their democratic rights.

Thaksin, by contrast, is loathed by the rival royalist "Yellow Shirts" backed by Bangkok's establishment, who accuse him of corruption and of lacking loyalty to the revered royal family.

Thirty-five countries have issued travel warnings for Thailand because of the protests, according to the country's tourism authority.

Analysts say the number of Red Shirts who actually rally on Sunday will be key to deciding whether they have any chance of pushing out the government before Thailand's next elections, due in December 2011.