Chad's President Idriss Deby, 68, died while commanding troops on the front line of a fight against northern rebels, an army spokesman said on Tuesday.
The announcement came a day after he was declared the winner of a presidential election for a sixth term.
The military said Deby had been commanding his army at the weekend as it battled against rebels who launched a major incursion into the north of the country on election day.
Deby, who was in power for four decades, "has just breathed his last defending the sovereign nation on the battlefield" at the weekend, army spokesman General Azem Bermendao Agouna said in a statement read out on state television.
The cause of death was not yet clear.
Deby’s son, General Mahamat Kaka, 37, was named the interim head of state and will lead the National Council of Transition, said Gen Agouna.
The military council now headed by Gen Kaka has already met to draw up a transitional charter, Gen Agouna said as he announced a curfew and border closures as well as the dissolution of the civilian government and elected Parliament.
"A call to dialogue and peace is launched to all Chadians in the country and abroad in order to continue to build Chad together," he said.
"The National Council of Transition reassures the Chadian people that all measures have been taken to guarantee peace, security and the republican order."
Deby's campaign said on Monday that he was headed to the front lines to join troops battling "terrorists".
Rebels based across the northern frontier in Libya attacked a border post on election day and then advanced hundreds of kilometres south towards the capital N'Djamena.
Deby was re-elected for a sixth term on Monday with 79 per cent, making him one of Africa's longest-serving leaders. Several opposition leaders boycotted the vote.
He took the title of "Marshal" last year and said before last week's election: "I know in advance that I will win, as I have done for the last 30 years."
Over the years, Deby survived numerous armed rebellions and managed to stay in power until this latest insurgency led by a group calling itself the Front for Change and Concord in Chad.
Deby was also dealing with mounting public discontent over his management of Chad's oil wealth and crackdowns on opponents.
Western countries have seen Deby as an ally in the fight against extremist groups, including Boko Haram in the Lake Chad Basin and groups linked to Al Qaeda and ISIS in the Sahel. French President Macron was one of the first world leaders to speak out on the President's death, saying France would stand by the nation of Chad and had, "lost a great soldier" and "brave friend" in Deby.
The US also expressed its condolences over the loss of Deby but emphasized their support for, "peaceful transition of power" in accordance with the Chadian constitution.
Deby, a herder's son from the Zaghawa ethnic group, came to power in a rebellion in 1990, when his rebel forces overthrew then-President Hissene Habre, who was later convicted of human rights abuses at an international tribunal in Senegal.
