Faustin Twagiramungu, seen to be the prime challenger to President Paul Kagame, said he would pull out of the race unless harassment of his supporters was stopped.
"It is not a democratic campaign because my people are being threatened … if they continue threatening my people or killing them, I will withdraw,"Twagiramungu, who is in the fray as an independent candidate, said.
A moderate Hutu, Twagiramungu had returned to Rwanda in June after almost eight years in exile in Belgium to contest the polls, the country's first since the 1994 genocide.
But the contender said President Kagame's form of democracy was politically repressive and would never unite Rwanda.
Rwanda is still struggling to recover from the legacy of the massacres in 1994, when ethnic Hutu extremists killed 800,000 minority Tutsis and moderate Hutus.
A Tutsi, Kagame has promised to share power if he won the elections scheduled for 25 August.
''It is not a democratic campaign because my people are being threatened…if they continue threatening my people or killing them, I will withdraw''
Twagiramungu, however, said he would not work with Kagame ever.
"I cannot, in capital letters, be a prime minister to President Kagame, but I can still play an important role as an opposition leader," he said.
Twagiramungu was prime minister in the first transitional government set up after the genocide. Kagame was vice-president at the time but already seen as the main power in the Rwandan Patriotic Front (RPF).
While extremist Hutus hated Twagiramungu for opposing the Hutu-sponsored carnage, he later fell out with the RPF when he accused it of reprisal killings of Hutus in the wake of the genocide.
The presidential vote, to be followed next month by parliamentary polls, is Rwanda's first multiparty election.