Supporters of the red-shirt United Front for Democracy against Dictatorship (UDD) have begun gathering on Ratchadamnern avenue near the Democracy Monument for a rally to remember the April 10, 2010 clash with government forces, reports said.
Around 11am they were on footpaths on both sides of the avenue, and a stage with sound equipment was being set up in the area.
Stalls seling souvenirs were all over the area, as seen in past rallies.
The actual UDD rally to commemorate the third anniversary of the bloody clash between government forces and red-shirt protesters began at 1pm, when two political parades moved out - from Phan Fa bridge and Sanam Luang - to meet at the Democracy Monument.
The rally was expected to continue until midnight.
Pol Col Rithikorn Saisanan na Ayutthaya, the Samranrat police chief, said four companies of police had been deployed to maintain law and order.