The rally was called by the Wakit Tamma, a coalition of opposition parties and civil society groups, to denounce the Transitional Military Council.
The TMC is headed by 37-year-old Mahamat Idriss Deby Itno, the son of the president who died in April fighting northern rebels after more than three decades in power.
Deby junior has promised “free and democratic” elections following an 18-month national dialogue process of reconciliation.
But he recently refused to rule out prolonging the transition period if “certain conditions” were not fulfilled.
Chadian authorities had approved Saturday’s march, but on a different route to that taken by the protesters.
The approved route, along a three-kilometre (two-mile) avenue in the city centre, “is very far from our base and it’s difficult to gather everyone in this place,” one opposition leader, Succes Masra, of the Transformateurs party, told AFP.
An AFP reporter said police fired tear gas to break up the protest in front of the Transformateurs headquarters, and Masra said several people were injured.
“We were violently dispersed,” said one demonstrator, Anasthasie, 28.
Police spokesman Paul Manga told AFP that “when the route is not respected and the demonstrators block traffic it creates problems for public order”.