The president says emergency powers will last for 15 days.
The decision came after Deby’s meeting with his French counterpart, Nicolas Sarkozy, in the Chadian capital of Ndjamena.
Sarkozy, on an African tour with wife, Carla Bruni, made a brief stop in Chad to persuade Deby to accept an international investigation into the alleged crackdown on political dissidents.
Weeks earlier France helped deliver the country from a rebel attack on its capital.
Rights groups have charged that the government declared a state of emergency on February 14 as an excuse to terrorise peaceful political opponents.
Sarkozy had said he wanted a “credible” international commission to probe the whereabouts of two political opponents who had gone missing since the coup attempt.
Opposition supporters claim Ngarlejy Yorongar and Ibni Oumar Mohamat Saleh who have been missing for weeks, were arrested by Chadian forces.
The government has confirmed arresting Lol Mahamat Choua, the country’s former president, as an “enemy combatant”.
He was held in a military prison before being placed under house arrest.
France plays a key role in a planned 3,700-strong peacekeeping force, known as Eufor, to protect refugees from Darfur in neighbouring Sudan, as well as others embroiled in conflicts along Chad‘s border.
Chad accused Sudan of backing rebels to destabilise the country.
Sarkozy said France “is available in the coming days to favour” mediation between the two neighbours.