Army commander Prayuth Chan-ocha on Friday challenged red-shirt leader Jatuporn Prompan to "accept responsibility" for accusing him of wanting to be interim prime minister.
Gen Prayuth was unclear by what he meant.
He said the public would see for themselves whether there was any truth in Mr Jatuporn's allegations, and demanded to know what the United Front for Democracy against Dictatorship (UDD) leader would do to if his comments were proven untrue.
Gen Prayuth's role has come under the spotlight from pro-government groups after he previously issued a stern warning that the army would use "full force" to handle any further political violence.
Some core UDD leaders said the warning indicated the army was preparing to stage a coup, paving the way for an interim premier as demanded by the anti-government People's Democratic Reform Committee (PDRC). The army has denied the allegations.
Mr Jatuporn said a reliable military source told him the top brass had met and agreed Gen Prayuth should be nominated as interim premier in four days.
He claimed Gen Prayuth would make the cut because he has worked with PDRC secretary-general Suthep Thaugsuban, former premier and opposition leader Abhisit Vejjajiva and former prime minister Yingluck Shinawatra, under various administrations.
But the army chief said he had been the subject of similar accusations before, questioning how Mr Jatuporn would "accept responsibility" for making the comments, and insisting the army has no control over how the political situation will develop.
Mr Jatuporn urged pro-government supporters to come out in full force to oppose the appointment of an interim premier at their rally site on Utthayan Road.
He said the red shirts would stay put at the protest site, unless the Senate makes a move to nominate an interim prime minister and suggested the Election Commission hold a referendum to let the people decide if they want reform before an election.
"The red shirts will be at Utthayan Road from May 17-19. We will stay there. If the Senate resolves to appoint an interim premier or do whatever the PDRC says, the red shirts will be mobilised," he said.
Meanwhile, the Internal Security Operations Command clarified Gen Prayuth's statement, saying the army chief was talking about invoking martial law when he threatened the use of full force.
Isoc spokesman Col Banphot Poonpian said Gen Prayuth was referring to a worst-case scenario and whether martial law would be enforced. He insisted the enforcement of martial law and a military coup were not related.
"It is a warning for all sides to refrain from violence. The army is concerned about the use of war weapons in attacks," he said.
He said the military hopes the situation will improve and martial law will not be necessary. He also said troops being deployed to step up security in Bangkok are operating under the Centre for the Administration of Peace and Order.
The coordination centre for the red-shirt movement in Uttaradit meanwhile announced it would mobilise red shirts from 17 provinces in the North to the Utthayan Road rally in Bangkok between Sunday and Monday.