Red shirt activists braving the rain gathered gradually at Democracy Monumnet on Ratchadamnoen Avenue, listened through the afternoon to UDD leaders addressing the group from a stage.
Most of the speeches involved with the September 19, 2006 coup, the political unrest in 2010, and unresolved questions for which there has been no clear solution on the violence.
UDD leader Weng Tojirakarn said that from 8-9pm tonight, former premier Thaksin Shinawatra may address the rally via video link.
Other UDD personalities scheduled to be on stage during the gathering include Veerakan Musikapong, Jatuporn Prompan, Nathawut Saikua and top UDD leader Thida Thavornseth.
The red shirt activists rallied on Ratchadamnoen Avenue, forcing traffic from Khok Wua intersection to Phan Fah bridge.
About 3,000 police officers were deployed around the rally site to maintain law and order.