Deby, 68, who came to power in 1990 rebellion, pushed through a new constitution in 2018 that reinstated term limits but would let him stay in power until 2033. His opponents accuses him of trying to create a monarchy.
In the capital N’Djamena, hundreds of protesters set tires on fire and chanted “No to a sixth term!” and “Leave, Deby!,” a witness said.
Police fired tear gas and made several arrests, including of Mahamat Nour Ibedou, a prominent human rights activist. Protests were also held in the cities of Moundou, Doba, Sarh and Abeche, witnesses said.
Chad is an ally of Western nations in the fight against Islamist militants in West and Central Africa and one of the largest contributors to the U.N. peacekeeping mission in Mali.
Deby has faced strikes and protests in recent years over economic woes caused by low oil prices and armed rebellions in the desert north, where former colonial power France has intervened in support of the government.
But he has drawn on his effective control of state media and institutions to maintain political dominance and won repeated landslide election victories.