The army immediately announced that Idriss Déby's son would head a military council for 18 months before elections were held.
Former colonial power France, which has a large military base in Chad, appeared to back the takeover for "stability" amid "exceptional circumstances".
Opposition parties have also condemned what they called a "dynastic coup".
Trade union have called for a general strike, while rebel group Fact said Chad was "not a monarchy".
HJ/PR