A group of 11 officers, led by Baradine Berdei Targuio, president of the Chadian Organisation of Human Rights, were behind the attempt, according to a government statement.

The security services arrested those responsible some time after December 8, the statement added.

A formal investigation had been opened for "violation of the constitutional order, criminal association, illegal possession of a firearm and complicity", said Communications Minister and government spokesman Aziz Mahamat Saleh.

The investigating magistrate leading the case has charged them and ordered their detention, he added.

"The investigation is following its course and the government means to do everything to shed light on this affair and to determine responsibility," Saleh said.

Targuio was sentenced in February 2021 to three years in jail on charges of undermining the constitutional order for having written that Chad's then leader, General Idriss Deby Itno, was seriously ill.

Deby, who died in April 2022, has been replaced by his son, General Mahamat Idriss Deby Itno.

Outlawed demonstrations were held on October 20 to mark the date when the military junta had initially promised to cede power -- a deadline that has now been extended by two years.

Around 50 people died, including 10 members of the security forces, according to an official toll.

But opposition groups say the real count was much higher, and allege that unarmed civilians were massacred.

Deby accused the demonstrators of "insurrection" and attempting to stage a coup.

The authorities have said 601 people, including 83 minors, were arrested in the N'Djamena area alone and taken to Koro Toro, a high-security jail located in the desert 600 kilometres (375 miles) from the capital.

A total of 401 people were put on trial in a court at the jail, proceedings that lawyers boycotted in protest.

After a four-day trial, 262 were jailed for two to three years, 80 were given suspended terms and 59 were acquitted, the prosecutor said.

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.