"This fight will be harder than any other ... You must think how we can deal with Suthep and those supporting him,'' Jatuporn Prompan, a UDD leader and senior member of the ruling Puea Thai Party, told thousands of cheering supporters in Nakhon Ratchasima, northeast of the capital.
It was unclear whether Jatuporn was calling for an armed struggle, but he was speaking just hours after gunmen shot at an anti-government protest stage and threw explosive devices in the Khao Saming district of the eastern province of Trat, killing a 5-year-old girl and wounding 41 people.
Week of protests
Anti-government protesters have blocked main Bangkok intersections for weeks with tents, tyres and sandbags, seeking to unseat Yingluck and halt the influence of her billionaire brother, Thaksin Shinawatra, an ousted former premier regarded by many as the real power behind the government.
The protests are the biggest since deadly political unrest in 2010, when Thaksin's "red shirt'' supporters paralysed Bangkok in an attempt to remove a government led by the Democrat Party, now the opposition.
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More than 90 people were killed and 2,000 wounded when Suthep, at the time a deputy prime minister, sent in troops.
Presenting a further headache for Yingluck, Thailand's anti-corruption body filed charges against her last week over a rice subsidy scheme that has left hundreds of farmers, her natural backers, unpaid.
Yingluck is due to hear the charges on Thursday.
The UDD, largely made up of Thaksin supporters based in the populous north and northeast, was formed in 2008 as a counter-force to the yellow-shirted anti-Thaksin People's Alliance for Democracy group.
Thanawut Wichaidit, a spokesman for the UDD, said a strategy to counter anti-government protests in Bangkok had yet to be worked out, but that the movement wanted to avoid civil war.
"We want to fight peacefully, without weapons, but we have not yet decided how we will proceed and that is why we are meeting today to come up with a plan,'' Thanawut told Reuters.
"The thing we are trying to avoid at all costs is a civil war and any kind of confrontation.''
The protests are the latest chapter in a political conflict that has gripped Thailand for eight years and broadly pits Bangkok's middle class and elite, and followers in the south against rural backers of Yingluck and her brother.
UDD chairwoman Thida Tawornseth said Sunday's rally would consolidate plans to restore democracy after the opposition boycotted and disrupted elections this month, leaving the country under a caretaker government. On Saturday, she ruled out any plans for violence.
Four protesters and a police officer were killed on Tuesday when police attempted to reclaim protest sites near government buildings. Six people were wounded by a grenade on Friday.