Tens of thousands of red-shirted supporters of Thaksin Shinawatra, the deposed prime minister, have gathered in Bangkok to demand the government's ouster, sparking a major security operation.
Riot police and soldiers guarded government offices as part of a 50,000-strong security force deployed on the streets to prevent any outbreak of violence on Sunday.
Bangkok police said 45,000 protesters from the north and northeast of the country had turned up at the rally site by Saturday night.
The government said it expected about 70,000 Red Shirts(the colour worn by Thaksin supporters) to attend, but the protesters have said the number will be several times higher.
"We will declare our demands to the government, that it must step down and dissolve the house," Jatuporn Prompan, a Thaksin supporter said.
"If our demands are not met then we will step up our campaign on Monday but I can reassure everyone that it will be peaceful."
Al Jazeera's Wayne Hay, reporting from Bangkok, said the security forces and the government are bracing themselves for a tense few days.
"They are calling it D-Day here in the Thai capital, Bangkok, the day that the supporters of the former prime minister Thaksin Shinawatra will try to force parliament to be dissolved and for fresh elections to be called," he said.
"Now if the demands of the so-called Red Shirts are not met, they say that they will spread out around the city and paralyse Bangkok. They are also saying that they will move onto what they are calling an important location; it's believed that location is a military base nearby, to try to apply direct pressure to the prime minister [Abhisit Vejjajiva].
"The Red Shirts claim that they can achieve their goal within four days and they still believe they can attract a million people to their protest. Realistically, the numbers are expected to be around the 100,000 mark," our correspondent said.
Vejjajiva, the Thai prime minister, has refused to bow to the protesters' demands and warned the public not to be complacent about the potential for violence.
Sunai Phasuk, a consultant for Human Rights Watch in Thailand, told Al Jazeera: "Vejjajiva has already made his decision that he is not going to surrender to pressure from protests. So we are now entering a period where both sides are locking horns."
The government has invoked a strict internal security act ahead of the demonstrations, allowing it to set up checkpoints, impose curfews and limit movements.
Thaksin, who was ousted in a coup in 2006 and has been living mostly in Dubai to escape a two-year jail term for corruption at home, has been encouraging his supporters to attend the rally using text messages and putting messages on his Twitter page.
The Red Shirts called the protests two weeks after Thailand's highest court confiscated $1.4bn of Thaksin's assets, frozen after the 2006 coup, saying that he had abused his power.
The Red Shirts argue that the government of Vejjajiva came to power illegitimately with the backing of coup leaders and the ruling elite, and they are demanding new elections.
The demonstrations are set to be the biggest since the Red Shirts held protests last April, which descended into violence in which two people died.
Opponents of the Red Shirts - the so-called Yellow Shirts - say that Thaksin is corrupt and fails to show sufficient loyalty to the revered royal family.