Thailand's pro-government "Red Shirts" began massing Saturday in Bangkok to challenge a bid by opposition protesters to install an unelected regime in power after the removal of the prime minister.
The dismissal of premier Yingluck Shinawatra and nine ministers by the Constitutional Court this week for the improper transfer of a top security official has plunged the restive kingdom deeper into crisis.
Officials said about 3,000 police officers were on stand-by for the pro-government rally on the western outskirts of Bangkok on Saturday, with turnout expected to peak in the evening.
"We are ready to fight," senior Red Shirt Kwanchai Pripana told AFP ahead of the gathering.
"We will not use violence but we will use the power of the masses to fight for democracy."
The Red Shirts have said they will keep up their protests for as long as necessary to defend the wounded administration.
Rival opposition demonstrators are gearing up to try to deliver a knock-out blow to the remnants of the government to enable an unelected leadership to take the reins of the Southeast Asian nation.
Such a move would infuriate supporters of Yingluck and her elder brother Thaksin Shinawatra, who was deposed by royalist generals in a coup in 2006, an event that ushered in years of political turmoil.
Political violence has left at least 25 people dead and hundreds wounded in gun and grenade attacks by shadowy assailants in recent months, mostly targeting opposition demonstrators.
The fear is that armed elements on both sides of the political divide could seek to incite further unrest.
Police used water cannon on Friday against anti-government protesters attempting to enter a state security agency.
The opposition activists also surrounded a number of television stations in a move decried by rights campaigners as media intimidation.
The anti-government protesters want the upper house of parliament — almost half of whose members are unelected — to remove the weakened cabinet including new Prime Minister Niwattumrong Boonsongpaisan, a Thaksin loyalist.
But critics say their call for the Senate to appoint a new prime minister has no legal basis.
Thai courts have removed three prime ministers linked to Yingluck's family, who have won every election completed since 2001.
The last round of polls held in February was voided after disruption by opposition protesters and the results were never announced.
Thaksin lives in Dubai to avoid jail for a corruption conviction that he says was politically motivated, but he is accused by his opponents of clinging to power through his political allies at home.
The billionaire tycoon-turned-politician is popular in the northern half of Thailand thanks to his policies aimed at rural voters, but hated by many southerners and members of the Bangkok-based elite and middle classes.
The government wants to press ahead with a planned July 20 election but the new poll date has yet to be endorsed by a royal decree and critics have accused poll officials of siding with the opposition.