'We are gathering to show we want to protect democracy,' Mr Chatuporn Prompan, a leader of the United Front of Democracy against Dictatorship (UDD), told the crowd.
The UDD rally will coincide with a march on Monday by the anti-government People's Alliance for Democracy (PAD).
The PAD are planning to demonstrate outside the national police headquarters against what they say was excessive police force against a PAD demonstration at the parliament building on Tuesday. Several policemen were shot in the clashes with armed PAD protesters.
PAD leader Pipop Thongchai said the demonstration would be peaceful and would demand that Prime Minister Somchai Wongsawat resign for ordering police to clear the protesters ringing parliament. -- REUTERS