UDD core members, most of them from the North and the Northeast - including Ubon Ratchathani, Mukdahan, Khon Kaen, Udon Thani, Chaiyaphum, Surin, Chiang Mai and Chiang Rai - were at the Liptapanlop Hall, the venue of the meeting.
UDD leaders who showed up early this morning included Tida Thawornset and her husband Weng Tojirakan, who went up on the stage to greet their supporters.
The meeting was expected to discuss the military's movement of tanks and artillery to Bangkok, which the UDD sees as unusual. The army says it is for the annual Children's Day exhibitions and the inaugural Army Day parade.
The UDD leaders were also expected to discuss their reaction to the People's Democratic Reform Committee's planned shutdown of Bangkok on Jan 13.
Jatuporn Prompan and Nattawut Saikuar were expected to arrive at the meeting venue in the afternoon.