BANGKOK, THAILAND - Thailand's government has vowed to get tough with protesters loyal to fugitive former premier Thaksin Shinawatra, as the capital braced for the biggest rally since bloody riots two months ago.

Bangkok police said more than 3,000 officers and 1,000 soldiers would be on hand to guard government offices, as they predicted up to 50,000 Thaksin loyalists could gather in the capital's historic quarter from 4pm (5pm Singapore time).

Deputy prime minister Suthep Thaugsuban, who travelled to Cambodia Saturday, said he had drafted a document to invoke an internal security law that gives more power to the army in case the rally turns sour.

But he said that the national police chief would oversee the situation for now and ensure protesters did not block access to government offices.

"The government will decisively enforce the law. We will make sure that all government buildings are not sealed off," Suthep, in charge of domestic security, said.

Major General Suporn Phansua told AFP that police estimated 30,000 to 50,000 protesters, mostly from Bangkok and surrounding provinces, would show.

The group, known as "Red Shirts" because of the colour they wear, said they would stay at the site until dawn on Sunday to demand that Prime Minister Abhisit Vejjajiva dissolves the House and calls fresh elections.

"I think hundreds of thousands of our red shirt protesters across the country will turn out for today's demonstration," protest leader Jatuporn Prompan told AFP.

Thaksin, currently living in Dubai to escape a jail sentence for corruption, is due to telephone Saturday's rally at around 8:30pm (9.30pm Singapore time).

"Thaksin... will talk about the government's failure to solve the economic crisis and may rebut the government's allegation (that the red shirts) plan to incite violence in the city," Jatuporn added.

The Red Shirts stormed a key Asian summit on the Thai coast on April 11, forcing its cancellation, before rampaging through the capital, leaving two people dead and 123 injured, and prompting Abhisit to declare emergency rule.

Protesters clashed with security forces in Bangkok over two days but finally dispersed after troops surrounded them and threatened to move them by force.

British-born Abhisit is currently on an official visit to China and due to return late Saturday.

Since Thaksin's ouster in a coup in 2006, Thai society has been deeply split between his supporters among the largely rural poor and the powerful Bangkok cliques in the palace, military and bureaucracy.

The kingdom has been wracked for months by rival rallies.

Opponents of Thaksin, known as "Yellow Shirts," staged their own protests last year that led to a nine-day blockade of Bangkok's airports and left more than 300,000 visitors stranded, badly denting the kingdom's tourist-friendly image.

Thaksin has made repeated addresses to his grassroots supporters in the kingdom's northeast in recent months, telling them he is homesick and wants to return to work in Thailand.