Jatuporn Prompan, an MP from the ruling People Power Party (PPP), said that about 100,000 people were expected to attend the rally, which was organised by the United Front for Democracyy Against Dictatorship (UDD).
"Thaksin ... will speak for an unlimited time on whatever topic he wants to talk," he said.
"This demonstration will be a peaceful show of force by those who support Thaksin and those who support democratic values in Thailand."
It will the former prime minister's first speech to supporters since he was sentenced in absentia to two years in jail for breaking corruption laws.
Several Thai newspapers reported that Thaksin had flown to Hong Kong to make the address by telephone.
Thaksin remains hugely popular among much of Thailand's rural population, but many in the cities revile him, accusing him of coruption and nepotism.
Somchai Wongsawat, the current prime minister and Thaksin's brother-in-law, has been under mounting pressure to resign since clashes between opposition supporters and police in October left two people dead and almost 500 others injured.
|Opposition supporters accuse Thaksin of corruption and nepotism [File: AFP]
Around 2,000 police officers were deployed at the Rajamangala stadium for the pro-government rally, but the opposition People's Alliance for Democracy (PAD) said it would not confront government supporters.
"The PAD will not move from its stronghold at Government House to confront the UDD [United Front for Democracy against Dictatorship]," Suriyasai Katasila, an organiser for the group, said.
"The PAD has stepped up security measures to the maximum," Suriyasai wrote on the group's website. "PAD will monitor closely the phone-in by Thaksin."
PAD demonstrators have been camped out in the grounds of the prime minister's offices since August. On Thursday, they marched to the British embassy to demand that Thakisn be extradited.
The Office of the Judiciary said it would monitor Thaksin's address to see if his remarks amount to contempt of court.