The regime is braced for unrest, and has prepared water cannon, sound-wave machines, teargas and batons in case protests turn violent. Government insiders say that there are also fears of gun and mortar attacks, and political leaders will be moved to a military command centre during the rallies.

Organisers are hoping to bring a million people on to the streets to protest against what they see as a regime — comprising a clique of military officers, allied with the “aristocracy” and the governing Democrat Party— that has unfairly brought down a series of populist governments, beginning with that of Thaksin Shinawatra, a former Prime Minister.

The Government has invoked the Internal Security Act, giving security forces the power to introduce curfews and restrict gatherings.

At a motorway checkpoint at Wang Noi, 40 miles (64km) northeast of Bangkok — and a potential flashpoint in the build-up to Sunday’s biggest rally in central Bangkok — police and soldiers clad in full riot gear searched cars, pick-up trucks and buses for weapons and checked passenger and vehicle documentation as people from the rural northeast converged to drive to the capital.

Three vehicles filled with of protesters from the United Front for Democracy against Dictatorship (UDD), dressed in their trademark red shirts and waving enormous flags, were eventually waved through.The protest began at red-shirt gatherings across the nation at 12.12pm yesterday, heralded by applause and the sound of Buddhist gongs and chanting.

At Lak Si, on the edge of Bangkok, about 2,000 red-shirt activists cheered and waved as their demonstration began. Chawarit Chawangkiet, 37, a computer programmer, said that he felt compelled to joined the protest because he wanted political reform. “I want to fight for democracy,” he said. “This country is controlled by invisible forces.”

The red-shirt leader Weng Tojirakarn, who was watching over the Lak Si gathering, told The Times that the aim was to fight for a system in which all people were treated equally, elections were called and the people allowed to decide the political future of the country. He said that there was an need to "end the double standard which is so backward".

UDD leaders say that hundreds of thousands of rural poor, especially, especially from the north and northeast, are expected to join the protest. Sean Boonpracong, a spokesman, said that the Government was doing everything it could to deter protesters from coming to Bangkok, including setting up huge checkpoints on highways, threatening to shut petrol stations, banning farm vehicles and out-of-state taxis and invoking the draconian ISA.

UDD leaders say that they want to change an age-old system of privilege that entrenches the wealth of the urban elite. “Our goal is dissolution of Parliament. If dissolution was announced tomorrow, there would be no
movement,” Mr Boonpracong said.

Bangkok’s Suvarnabhumi airport is on alert with authorities determined to avoid a repeat of its occupation in 2008, when royalist yellow-shirt supporters, aligned with the Government, seized it in protest. Mr Thaksin, speaking from exile in Dubai, urged supporters to join the protest. He is expected to address the rally via video-link.Abhisit Vejjajiva, the Prime Minister, has said that fresh elections, and his resignation, were possible to quell dissent. “I will not hold on to power,” he told Parliament this week.