
Core leader Jatuporn Promphan said that up to 300,000 people were expected to join the movement and pressure Prem to quit.
Besides the main demonstration site at Government House, the protesters plan to rally in the Royal Plaza and outside of the nearby Si Sao Thewes residence of Prem.
The red shirts have accused the Privy Council president, who is His Majesty the King's chief adviser, of being the mastermind behind the September 19, 2006 coup that overthrew the Thaksin Shinawatra government, a charge denied by Prem.
Jatuporn said Thaksin, a fugitive in exile, would be on standby throughout the day to join protesters via telephone and video link.
In last night's address to the red shirts, Thaksin called on people who wanted "real democracy" to gather in great numbers at today's rally.
He insisted on his loyalty to the monarchy, "which has been in my blood since my childhood".
The ousted leader also criticised former aide Newin Chidchob, the banned politician who yesterday accused Thaksin of causing trouble for the country.
"This afternoon I saw a soap opera. They made the villain a hero, and they let the villain lecture the hero," Thaksin said.
Protesters yesterday were flocking to Bangkok from upcountry with food and other supplies, because they expect today's rallies to be lengthy.
Meanwhile, security officials have been instructed to brace themselves for an uprising, because the red shirts may besiege provincial halls across the country.
In Bangkok, about 30 police officers were deployed yesterday to guard the road leading to Prem's Si Sao Thewes residence.
The red shirts have been threatening to lay siege to his residence since they began rallying late last month.
In the northeastern province of Ubon Ratchathani, 500 red-shirt protesters left for Bangkok yesterday in 40 vehicles fully loaded with supplies.
In Sakon Nakhon, six buses full of red shirts departed for the main rally site at Government House.
At least 200 police officers are on standby in case of violence in Chiang Mai, where the red shirts are also strong.
The police chief of the northeastern province of Nakhon Ratchasima ordered a checkpoint to be set up, to prevent protesters from reaching Bangkok.
Meanwhile, more than 2,000 People's Alliance for Democracy (PAD) supporters gathered at Krabi's city hall yesterday, voicing their intention to protect the monarchy.
In Si Sa Ket, a group of government supporters attacked ousted prime minister Thaksin for making accusations against Prem.
Group leader Niyom Rodniam called on Si Sa Ket residents to think carefully before joining the red shirts, saying they could fall victim to ill-intentioned people.