Idriss Deby Into, who had just won re-election as president of Chad, is dead after fighting with rebel forces, military spokesman Azem Bermandoa Agouna says.
"The president of the republic, head of state, supreme chief of the armed forces, Idriss Deby Itno, has just taken his last breath on the battlefield in defence of territorial integrity," Agouna said.
The circumstances of his death were not immediately released.
Mahamat Idriss Deby Itno, the son of the late president and a four-star general, is to lead an 18-month transitional military government, Agouna said.
The government and the National Assembly have been dissolved.
Instead of the current constitution, a transitional charter will be announced shortly.
A 14-day mourning period was also imposed.
Air and land borders have been closed until further notice and a curfew is in effect between 6pm and 5am.
Chadian soldiers have been clashing with armed rebels fighting for the Front for Change and Concord in Chad (FACT), many of whom are believed to have crossed into the country from neighbouring Libya in time for the April 11 presidential election.
In recent days, the fighters have been trying to advance towards the capital N'Djamena but faced heavy resistance from Chad's army.
FACT is a military and political rebel movement founded in 2016 that has sought to destabilise the government of Deby, who kept a firm grip on power for 30 years.
The announcement of Deby's death at the age of 68 came a day after electoral authorities in the central African country said he won a sixth term by garnering nearly 80 per cent of the April vote.
Australian Associated Press