The military authorities have called the vote as a vital stepping-stone to elections next year — a long-promised return to democratic rule after they seized power in 2021 when President Idriss Deby was killed on the battlefield during a conflict with insurgents.
The proposed constitution would establish autonomous communities with local assemblies and councils of traditional chiefdoms among other changes.
But some of the political opposition and rebel groups have called for a "No" vote or said they will boycott the poll. They question the independence of the election commission and reject the new constitution for not engaging with their wishes including for federalization.
Decades of instability and economic mismanagement have hampered development in the oil-producing central African country, where nearly 40% of its 16 million people depend on humanitarian aid.
Brice Nguedmbaye Mbaimon, who coordinates a coalition voting "No," said Chad had experienced a unitary state for over 50 years without tangible progress.
"It is time to let the population organize into federated states and steer their own development," he told Reuters.
Haroun Kabadi, coordinator of groups voting "Yes," said the new constitution does offer more independence as it would allow Chadians to choose their local representatives and collect local taxes for the first time.
"These people talking about a federation simply want to divide Chadians into micro-states and fuel hatred between communities," he said by phone.
Meanwhile others are calling for a boycott, including former Prime Minister Albert Pahimi Padacke, who told Reuters the junta had too much control over the referendum process.
"This is not fair, and it is not democratic. That's why we have called on Chadians not to participate in this farce."
The Africa-focused Institute for Security Studies has warned that tensions around the vote could lead to a repeat of unrest that saw scores killed by security forces amid pro-democracy protests in October 2022.
A central concern is that the referendum could help cement the power of junta leader, Deby's son Mahamat Idriss Deby, who has already extended a proposed 18-month transition to democracy.
"The pattern of delay and obfuscation echoes the long-honed tactics of Idriss Deby who came to power by force in 1990 and then held on to it for three decades," analysts at the Africa Center for Strategic Studies said.
Around 8 million Chadians are registered to vote in the referendum. Provisional results are expected to be announced on December 24.
The military regime in Chad is one of several juntas in West and Central Africa, which has seen eight coups since 2020, sparking concerns of a democratic backslide in the region.