The killing of a former commander of general intelligence and his son by unidentified armed men prompted an official order last week "to secure the city of N'Djamena" and carry out "systematic searches" for weapons.
Heavily armed soldiers were deployed in parts of the capital to conduct the searches.
On Saturday, state television announced the replacement of the security minister under a slight government reshuffle.
Opposition parties announced last week that they would refuse to take part in the December elections, denouncing a "harmful climate of dictatorship and of terror".
"The country is heading towards the final crowning of the regime's complete illegitimacy after a bogus referendum that imposed a rejected constitution and a disastrous presidential election whose results were never recorded, compiled and counted," they said in a statement.
In mid-September, the World Organisation Against Torture warned of an increasing number of arrests and detentions without due process by the Chadian intelligence services.