Thousands of red-shirted protesters gathered in Bangkok amid tight security on Saturday, the third anniversary of a military coup against former prime minister Thaksin Shinawatra.
The "Red Shirt" movement of Thaksin supporters wants current prime minister Abhisit Vejjajiva to resign and hold elections.
The last time they took to the streets in large numbers was in April, when the protests turned violent - buses were burned, streets were blocked and the army was sent in to restore calm.
The leaders of the red-shirt movement - the United Front for Democracy Against Dictatorship - say they will ensure the protest does not turn violent.
But Mr Abhisit is taking no chances and has given the military special powers under an Internal Security Act. More than 9,000 soldiers and police have been deployed to guard key locations.
Mr Thaksin is in self-imposed exile after being convicted on conflict of-interest charges.
Border clash with Yellow Shirts
Meanwhile, "Yellow Shirt" protesters - supporters of Mr Abhisit -clashed with police and villagers near an ancient temple on the Cambodian border at the centre of a dispute between the two countries.
Demonstrators broke through barricades and were moving towards the 11th-century Preah Vihear temple, the scene of several deadly battles between Thai and Cambodian troops over the past year.