[Photo: Soldiers take part in a drill outside Government House in Bangkok September 19, 2009.]

BANGKOK - Thousands of red-shirted protesters rallied in Bangkok amid tight security Saturday to mark the third anniversary of a military coup against former prime minister Thaksin Shinawatra.

The September 19, 2006 putsch plunged the kingdom into three years of political turmoil which shows little sign of ending, with supporters of the exiled Thaksin leading the latest round of protest and counter-protest.

The government has imposed a draconian internal security law in the centre of the capital for the latest demonstrations and deployed more than 9,000 soldiers and police to guard key locations.

The "Red Shirt" movement says it wants current prime minister Abhisit Vejjajiva to resign and hold elections, raising concerns of a repeat of massive anti-government riots that hit Bangkok in April.

Police estimated that around 1,500 protesters had arrived at the protest site in the city's Royal Plaza and that many more were on their way for the official start of the demonstration at 1 pm (0600 GMT).

Red Shirt organisers had set up shops selling food and the movement's signature foot-shaped plastic clappers near a stage where protesters were set to gather in the evening to watch a video or hear a telephone address by Thaksin.

Deputy Prime Minister Suthep Thaugsuban said that unidentified groups of troublemakers could set off bombs in Bangkok and create unrest during the protest.

"What worries us is a third party trying to incite the unrest. Many groups have been ordered to act but I do not mean Red Shirts," he told reporters.

Tensions rose further when around 5,000 rival, anti-Thaksin "Yellow Shirt" protesters headed towards an ancient temple on the disputed northeastern border with Cambodia, officials said.

Soldiers, police and local villagers set up checkpoints to stop protesters travelling in buses and cars towards the 11th century Preah Vihear temple amid fears that their actions could spark conflict with Cambodia.

The movement wants the government to push out Cambodian forces and was trying to negotiate with authorities to allow them into the area, where there have been several deadly military clashes in the past year.