Chad's president Idriss Deby has been killed in a battle against rebels in the north, authorities said.
His death was announced a day after he was declared winner of a presidential election that would have given him a sixth term in office.
Most of the opposition boycotted the vote.
Deby ruled his country for more than 30 years and was an important Western ally in the fight against Islamist militants in Africa.
His son, Mahamat Idriss Deby, was named interim president by a transitional council of military officers, army spokesman Azem Bermendoa Agouna said on state television on Tuesday.
Deby, 68, took power in a rebellion in 1990 and was one of Africa's longest-ruling leaders, surviving numerous coup attempts and rebellions.
His death, the exact cause of which was unclear, could deepen Chad's problems, as well as those of its allies.
On the domestic front, the military is divided and the opposition bridling against years of repressive rule.
Internationally, France and the United States will be hoping their counterterrorism efforts are not now pushed off course.
France said it had lost a "brave friend" and Chad "a great soldier".
The White House offered "sincere condolences" to the people of Chad and supported "a peaceful transition of power in accordance with the Chadian constitution."