Red-shirt leaders and local officials have already played down an alleged call for the establishment of a "Sor Por Por Lanna", or "Lanna republic", ahead of the mass demonstration at the International Convention Centre in Chiang Mai.
The secession issue was said to have been raised by several red-shirt groups including Chiang Mai 51 Group led by Phetchawat Wattanapongsirikul and picked up by Pheu Thai leader and caretaker Interior Minister Charupong Ruangsuwan.
It has left the caretaker government in an awkward position and prompted army commander Prayuth Chan-ocha, deputy chief of the Internal Security Operations Command (Isoc), to take action.
Mahawan Kawang, a co-leader of the red shirts in Chiang Mai, said yesterday that today's demonstration will be a show of support for the caretaker government and caretaker pr Yingluck Shinawatra who have been under attack from the People's Democratic Reform Committee (PDRC) in recent months.
"The rally will offer moral support for the government. It will show the people who are trying to topple the government that the government is not alone," Mr Mahawan said.
He said the rally will serve the same purpose as last weekend's rally in the Northeast. He said many people who have never taken part in red-shirt activities in the past will also attend because they feel sympathy for the government.
Thousands of red shirts across the Northeast gathered in Thung Sri Muang in Udon Thani province last Saturday before a procession of leaders and supporters moved on to Kalasin, Maha Sarakham and Khon Kaen. The rally was aimed at countering the PDRC's campaign.
Mr Mahawan said core red-shirt members will attend the rally and state authorities have been warned about possible traffic chaos. Tens of thousands are expected to turn up.
Asked about banners with messages allegedly supporting separatism that recently appeared in the Chiang Mai municipal area, Mr Mahawan said the issue should not be taken seriously.
"It is totally impossible. No one took Mr Phetchawat's words seriously. He was furious. It is a joke and people in the North know this. They just laugh when they hear about it [a Lanna republic]. It can easily be proved [if it was for real],'' he said.
Pannawat Nakmool, a UDD leader in Uttaradit, said the red shirts simply aim to show their support for Ms Yingluck and her administration, who won power through democratic means.
Siriwat Jupamattha, a coordinator for the UDD in Phayao, said the Lanna republic controversy will not be raised at the rally since it is important the whole issue is not politicised further.
Chiang Mai governor Wichian Wutthiwinyu also played down talk of northern separatism.
''It is a political thing. The people of Chiang Mai know it is totally impossible. When it was politicised, it was blown out of proportion. I lodged a complaint with police as soon as the Isoc deputy chief ordered it done,'' the governor said.
He warned against discussing the matter, saying it could lead to serious problems.
However, Mr Wichian had some security concerns for today's rally and has ordered that security be stepped-up for red-shirt protesters.
The red shirts intend to gather in Lampang before moving on to Chiang Mai. The Chiang Mai governor estimates the crowd will total about 10,000, while red-shirt leaders say the event should attract at least 40,000.
Thanawat Yodchai, mayor of Mae Hiah, said the Lanna republic issue is nothing more than a frustrated man's talk.
It is more akin to a sarcastic remark and if it sounds like a threat, it is an empty threat, he said.
Local residents told the Bangkok Post that the very idea of secession was a joke.
A senior journalist said it takes huge resources to establish a new state and it is something mentioned in passing by some red-shirt members. He thinks it is how some red shirts vent their frustration.
A state official, who asked not to be named, said not all people in Chiang Mai are allied with the UDD. They have not taken to the streets to oppose UDD activities because they choose to avoid confrontation, he said.
Thanet Charoenmuang, a former political science lecturer at Chiang Mai University, dismissed the idea of secession as an impossible dream. However, he said the very idea of Chiang Mai being the red-shirt capital is logical because the city and province is home to the Shinawatra family and a red-shirt stronghold. The province is rich in culture and history, he said.
Wanchai Jomthan, 57, leader of Sor Por Por Lanna, (Sor Por Por Lanna stands for Samatcha Pokpong Prachatippatai Lanna, or Lanna Assembly for Defence of Democracy, said the group's sole purpose is to defend democracy, especially against a coup. He said his group has not yet decided whether to attend today's UDD rally.