They demand that incumbent Prime Minister Abhisit Vejjajiva to resign and hold elections.

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 in the capital.

Police estimated that around 5,000 protesters had arrived at the protest site in the city's Royal Plaza by 0600 GMT and that many more were coming from around the country.

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

Powerful army chief Anupong Paojinda on Friday denied rumours that the military was about to stage a coup against Mr Abhisit, who is heading to New York at the weekend for the UN General Assembly.