Deby, a close Western ally, was killed in battle with rebels on Monday, raising concern in the US and France that turmoil and uncertainty following his death will hamper the fight against militants across the Sahel region.
Chad has staged a state funeral for veteran ruler Idriss Deby Itno, a key figure in the fight against the Sahel's militant insurgency, as France and regional allies voiced backing for his son and successor, Mahamat Idriss Deby.
The elder Deby, who had ruled the vast semi-desert state with an iron fist for 30 years, died from wounds sustained fighting rebels at the weekend, the army said on Tuesday.
His death has stunned the Sahel and its ally and former colonial ruler France, battling a militant revolt that in nine years has swept across three countries.
The unrest has claimed thousands of lives and forced hundreds of thousands from their homes.
France backs son
Deby's coffin, draped in the national flag and surrounded by elite troops, was driven on the back of a pickup truck to the Place de la Nation square for ceremonies attended by foreign leaders, including French President Emmanuel Macron.
"France will never let anyone, either today or tomorrow, challenge Chad's stability and integrity," Macron said at the ceremonies.
"France is also there to sustain the promise of a peaceful Chad," he said.
But Macron also called on the newly appointed military government headed by Deby's son Mahamat Idriss Deby to foster "stability, inclusion, dialogue, democratic transition."
There was a 21-gun salute for Deby, who only last August had been declared a field marshal — the first in Chad's history — after leading an offensive against militants in the west of the country.
Macron and his counterparts from Burkina Faso, Mali, Mauritania and Niger jointly met earlier with Deby's son.
The leaders, expressing a "unity of views", said they "stood by Chad and expressed their joint support for the process of civilian-military transition, for the stability of the region," a French presidential official said.
READ MORE: Who is Mahamat Deby, Chad’s new leader?
'Free and democratic' election
The 37-year-old general was named president and head of a military council immediately after Deby's death was announced.
He will wield full powers but has promised "free and democratic" elections after an 18-month transition period that can be extended once.
The move has been branded an "institutional coup" by the opposition.
Macron, in his tribute to the fallen president, said "you lived as a soldier, you died as a soldier, weapons in your hands."
Deby's death was announced the day after he was declared the winner of an April 11 election — giving him a sixth mandate after three decades at the helm.
The army said the 68-year-old had died on Monday from wounds suffered while leading troops in battle against rebels who had crossed from Libya.
The Front for Change and Concord in Chad (FACT) has vowed to pursue its offensive after a pause for the funeral, with spokesperson Kingabe Ogouzeimi de Tapol telling AFP that the rebels were "en route to NDjamena."
Allies of the late leader had moved swiftly to ensure power remained in their hands, installing the younger Deby, whose nickname is "Kaka," as president and head of a transitional military council while dissolving parliament and the government.
The younger Deby until now had commanded the top-notch Republican Guard.
His father seized power in a chronically unstable country in 1990 and had twice thwarted attempted coups with support from France.
He was repeatedly returned to office in elections denounced by opponents as fraudulent.
But he gained a reputation in the West for his reliability in the fight to roll back militants, whose campaign has shaken the vast, impoverished region.
Chad has well-respected armed forces and hosts the headquarters of France's 5,100-strong Barkhane anti-militant mission.
It also partners with Burkina Faso, Mali, Mauritania and Niger in a regional anti-militant coalition called the G5 Sahel.
Deby's remains will be flown a thousand kilometres east to the village of Amdjarass near the Sudanese border, where he will be buried alongside his father close to his birthplace of Berdoba.