The group, known informally as the 'Red Shirts' but officially called the United Front for Democracy Against Dictatorship (UDD), would rally on Sunday in the capital, a leader said.
'We call on our group to come to rally at 4pm (5pm Singapore time) on Sunday at first and the rally will continue through to the next day,' Jatuporn Prompan, a government lawmaker, told AFP.
'Our supporters will come from across the country to Bangkok as the battleground is here,' he said.
Mr Jatuporn asked supporters to gather at Sanam Luang plaza, a popular protest spot in Bangkok's government district.
The site is not far from the prime minister's cabinet offices, which have been occupied by protesters from the anti-government People's Alliance for Democracy (PAD) since late August.
The PAD - who wear yellow shirts and headbands in what they say is a symbol of their desire to protect Thailand's revered monarchy - are locked in a stand-off with police at Bangkok's two main airports.
Clashes between the pro and anti-government groups have broken out sporadically across Thailand recently and people fear a large scale confrontation.
The pro-government side are supporters of ousted and exiled prime minister Thaksin Shinawatra - the object of the PAD's wrath, as they claim the current government is doing his bidding.
In early November, Thaksin gave a telephone address to a gathering of about 100,000 supporters on the outskirts of Bangkok, in which he said 'the power of people' could bring him back from exile. -- AFP