Members of the United Front for Democracy Against Dictatorship (UDD) in eight upper northeastern provinces met today in Udon Thani, which is the starting point.
Apart from Udon Thani, the people were from Nong Khai, Loei, Nong Bua Lam Phu, Bung Kan, Nakhon Phanom, Mukdahan and Sakon Nakhon.
From Udon Thani, they will march to Kalasin, Maha Sarakham and then Khon Kaen.
Chairwoman Thida Thawornseth said the red shirts met here today to begin their march to meet "people who do not love democracy" in Bangkok.
"Udon was picked in honour of the Rak Udorn Group of Udon Thani province, and its chairman Kwanchai Praipana, who is recuperating [from gunshot wounds]," she said.
At least 20,000 will begin the journey tomorrow and join with people who share the same ideology along the way.
The following week the UDD will go to Chiang Mai and stay for two nights.
"We will explain to the northern and northeastern people the need to do something before more damage is done to the country.
"This is because Thailand does not have only the People's Democratic Reform Committee. There's another group with even more people ready to make a move to let the country move on," she said.
She did not rule out the possibility that the red shirts would come to Bangkok but said it would depend on the situation.
"Democracy lovers are enraged as they have been targeted for a long time," she said.
As for talks of segregation, she said the UDD allows freedom of speech but everyone had to bear in mind this was not a resolution of the UDD's leaders.
"Why would we divide the country? After all, we are the majority of this nation. We won't allow the minority to dictate the fate of the majority," she said.
On the PDRC's decision to scale down its rally sites, Mrs Thida said it was for good reasons since the numbers were dwindling and there had been more threats in various forms. However, they keep their goal of shutting down businesses linked to the Shinawatras.
Vorachai Hema, a UDD leader, said the caravan would be 20km long.
"A march to Bangkok is possible if an elected government is toppled and not only by a coup. If independent organisations, courts or any invisible power try to topple the government, we need to stand up to them."