The mass anti-government rally, led by Thailand's United Front for Democracy against Dictatorship (UDD), or the red-shirted group, kicked off in the capital Bangkok on Saturday afternoon.
The red-shirted protesters are demanding the government to dissolve the House and hold a new general election.
The UDD protesters started gathering at the Royal Plaza and will later march to the Si Sao Thewes residence of the president of the Privy Council, General Prem Tinsulanonda.
The protest is also organized to mark the third anniversary of the Sept. 19, 2006 military coup, which brought down former Premier Thaksin Shinawatra from power.
The UDD group has long accused General Prem of being behind the military coup three years ago.
Ousted by the coup, Thaksin was accused of corruption, and kept in exile since then. In February 2008, Thaksin returned to Thailand to face corruption charges, but he later fled into exile again and was convicted in absentia.
Meanwhile, Thaksin has been perceived as the real leader of the UDD people.