A large number of red-shirt activists began gathering on Saturday at the Bonanza resort near Khao Yai to voice their support for constitutional change.
The members of the United Front for Democracy against Dictatorship also intend to express their opposition against a possible military coup now that the Pheu Thai government is moving ahead with a referendum on rewriting the 2007 charter.
UDD co-leaders including Jatuporn Prompan and former prime minister Somchai Wongsawat are scheduled to give speeches on the rally stage later in the evening.
Also expected to speak is Pheu Thai list MP Korkaew Pikulthong, who is accused of terrorism related to the 2010 unrest but was released from jail this week because he has immunity when Parliament is in session.
Fugitive former prime minister Thaksin Shinawatra is scheduled to phone in to talk to his red-shirt supporters about 8.30pm.
A total of 700 police were deployed at Bonanza and nearby areas in Pak Chong, Nakhon Ratchasima to maintain peace and order.