Mr Suthep, People's Democratic Reform Committee (PDRC) secretary-general, called on people countrywide to gather in a massive rally in Bangkok on Wednesday to oust caretaker Prime Minister Yingluck Shinawatra and her government.
Centre for Maintaining Peace and Order (CMPO) director Chalerm Yubamrung has announced the CMPO will reopen Government House on the same day for Ms Yingluck to work after over two months of shutdown because of the protests.
"It will be a large gathering of PDRC supporters," Mr Suthep told demonstrators at the Silom rally stage.
"We will fight together in a decisive battle."
He did not explain the battle details.
Feb 19 is also the day when many farmers, disgruntled by the overdue payments from the government under the rice-pledging scheme, will gather in Bangkok and the Civil Court will rule whether the emergency decree imposed by the government is legitimate.
Mr Suthep said he will tomorrow lead protesters from Pathumwan intersection in a march toward Government House to further reinforce the blockade around Government House to prevent Ms Yingluck from getting in.
Another group of PDRC supporters, led by ex-Democrat Thaworn Senneam, will rally at the Interior Ministry to prevent any attempt by the CMPO to reclaim the rally site there.