After three days in Laos, Thaksin is due to visit the Cambodian city of Siem Riep to give a speech on Saturday evening at a gathering attended by several leaders of the Red Shirt movement, the organisation said.
"Tens of thousands" of supporters in total are expected to greet Thaksin during his trip to the two countries over the Thai New Year, said Red Shirt spokesman Worawut Vichaidit.
"They will probably travel together and rent buses by themselves," he added.
Mass rallies by Thaksin's supporters in the Thai capital in 2010 descended into the kingdom's worst political violence in decades, with more than 90 people -- mostly civilians -- killed in a military crackdown.
His sister Yingluck Shinawatra became prime minister last year after a resounding election victory by the ousted premier's political allies.
Thaksin, who maintains warm ties with Cambodian Prime Minister Hun Sen, was toppled by royalist generals in 2006 and lives in Dubai to avoid a two-year prison sentence for corruption that he contends is politically motivated.
Hun Sen said in a speech on Monday that "perhaps more than tens of thousands" of Thaksin supporters would visit to hear the ex-leader who "cannot enter his motherland".
Thaksin remains a much-loved figure for many poor Thais, but he is hated by much of the Bangkok-based elite in military, palace and bureaucratic circles who see him as autocratic and a threat to the monarchy.
The former leader, who frequently visits other Asian countries, has downplayed suggestions that his political allies are seeking to clear the way for his return to Thailand without going to jail.
After his speech, Thaksin is due to travel to the Cambodian capital Phnom Penh on April 16 to play golf with Hun Sen.