A total of 401 people had been put on trial in Koro Toro prison, a high-security jail located in the desert 600 kilometres (375 miles) from the capital Ndjamena.
Lawyers had boycotted proceedings on the grounds of legality.
Of the others, 80 were given suspended terms and 59 were released, prosecutor Moussa Wade Djibrine said.
The trial ended on Friday after four days, with only state TV having the right to provide coverage, and the sentences were announced on Monday after the prosecutor returned to the capital.
Charges facing the defendants included taking part in an unauthorised gathering, destroying belongings, arson and disturbing public order.
Around 50 people, including 10 members of the security forces, died when police opened fire on demonstrators in N'Djamena and several other cities on October 20, according to an official toll.
But opposition groups say the real count was much higher, and allege unarmed civilians were massacred.
The protests had been called to mark the date when Chad's ruling military had initially promised to cede power -- a timeline that has now been extended by two years.
Strongman General Mahamat Idriss Deby Itno accused the demonstrators of "insurrection" and attempting to stage a coup.
Deby, 38, took power when his father, Idriss Deby Itno, who had ruled the arid Sahel state for 30 years, died during an operation against rebels in April 2021.
The authorities had previously said that 601 people, including 83 minors, were arrested in the Ndjamena area alone and taken to Koro Toro, a remote location two days by road from the capital.
Around 80 minors held there have been taken back to N'Djamena, the prosecutor said.