"We shouldn't provoke the army council by trying to deprive them of their legitimacy, by depriving them of their positive role in the revolution," National Umma Party chief Sadiq al-Mahdi told AFP in an interview.
"We must not challenge them in a way that makes it necessary for them to assert themselves in a different way," the veteran politician said.
Mahdi's elected government was toppled by now deposed president Omar al-Bashir in an Islamist-backed coup in 1989.
Since then Mahdi has fought Bashir politically, and in January threw his weight behind the protest movement that finally saw the army oust Bashir on April 11.
But since then the army has resisted transferring power to a civilian government as demanded by the protesters, who have camped in their thousands outside the army's Khartoum headquarters in a round-the-clock sit-in.
Mahdi's warning comes amid a deadlock in talks between the protest leaders and the 10-member army council over the forming of a joint civilian-military body that will replace the existing army body.
Leaders of the Alliance for Freedom and Change, the umbrella group leading the protest movement, insist the army generals are not serious about handing power to civilians.
Protest leader Mohamed Naji al-Assam said on Tuesday that the military had been seeking to "expand its powers daily".
In a move to step up pressure on the army rulers, the protest movement's leaders have called for a "million-strong march" on Thursday.
The military has been pushing for a 10-member joint civilian-military council including seven army representatives and three civilians.
Protest leaders want a majority of civilians on a 15-member joint council with seven military representatives.
"I think there are some signs that some of them (the army) have been provoked by some statements from the opposition that seem to belittle their role," said Mahdi.
"If we provoke the... armed forces which contributed to the change, we would be asking for trouble," said Mahdi, dressed in a traditional Sudanese turban and robe.
Mahdi expressed optimism the military would transfer power.
"They will hand over executive power to a civilian government if we present a credible, viable form of a civilian government," he said, speaking at his residence in Omdurman, the twin city of Khartoum across the Nile.
This, he said, was "because they know if ultimately they settle for a military dictatorship, they will be in the same position as Bashir."