Mugabe told supporters that his challenger Morgan Tsvangirai of the opposition Movement for Democratic Change would turn the country back over to white control if he won the runoff, according to CNN.
The veterans were not prepared to recognize a Tsvangirai victory, said Mugabe, who has ruled the African nation since its independence from Britain in 1980.
"They said they got this country through the barrel of gun, so they cannot let it go by a ballot," Mugabe said Thursday at a campaign rally in Murehwa.
Mugabe has remained immensely popular with the former militias, in part through a controversial program of seizing white-owned farms and turning them over to black soldiers who fought.
Tsvangirai, human rights groups and Western countries said Friday's warning was just the latest example of Mugabe's campaign of violence and intimidation leading up to the runoff.
The opposition leader said 66 of his followers have been murdered, but Mugabe countered that the opposition has been causing the violence.
Tsvangirai has been detained four times in the past week, and most recently, Zimbabwean police impounded two of his campaign buses, Reuters reported.
The Movement for Democratic Change's No. 2 leader, Tendai Biti, was arrested while returning to Zimbabwe on Thursday and charged with treason, which can carry the death penalty.
A judge ordered police to bring Biti to court Saturday and explain why he should not be immediately released, an opposition lawyer said Friday, according to the Associated Press.
The United States and other governments pointed to the arrest as proof the run-off election will not be free and fair.
In 2004, Tsvangirai was acquitted after a treason trial that lasted more than a year.