"We will never contemplate or support a military coup d'etat for changing the regime by force of arms," Turabi told reporters in Khartoum.
Turabi, 73, a former ally of Sudanese President Omar al-Beshir, was freed from detention on June 30, nearly 15 months after Beshir jailed him for allegedly sponsoring a coup.
He has been a vocal critic of the national unity government established in line with a January 9 peace deal that ended the more than two decades of south-north conflict that left some two million people dead.
"This so-called national unity government ... well it is a nice name and everybody likes to have a nice name, but in fact it is not worth the name," Turabi said.
His Popular Congress Party refused to come on board and formed an alliance with nearly a dozen other opposition parties, including the Umma party of former Prime Minister Sadiq al-Mahdi.
The peace agreement Khartoum signed with the former southern rebel group, the Sudan People's Liberation Movement, calls for the convening of general elections in four years.
"We have decided to keep away from both the executive and legislative bodies of this government and to lead a peaceful, nonviolent opposition until the general elections in which we will run and which we just hope will be free and fair," Turabi said.
Turabi's now defunct political group, the National Islamic Front, was behind the 1989 military coup that brought Beshir to power and he was believed to be the real power behind the regime until he had falling out with Beshir.