Suspected Boko Haram jihadists blew themselves up during a police raid in Chad's capital early on Monday, killing five officers and six militants, police and government officials said.
The raid of an illegal arms cache was part of a security operation aimed at dismantling a network of Boko Haram militants blamed for twin suicide bombings in N'Djamena on June 15.
The apparently synchronised attacks were the most deadly that Western ally Chad has known and killed 34 people.
"After an interrogation, a suspect showed us a house for building bombs," said interior minister Abderahim Bireme Hamid. "When we arrived on the scene at 5am our forces came across the ... terrorists who blew themselves up."
A police officer who participated in the raid said one of the militants was wearing an explosive belt and detonated it as police arrived.
A Reuters witness outside the house in N'Djamena's eastern neighbourhood of Diguel saw Red Cross workers load at least four white bodybags onto a truck. The torso of one of the dead lay in the sand outside the house.
Prosecutor Alghassim Khamiss said the house was an arms cache and that authorities had seized a large number of home-made explosives and eight mortars there.
Security forces later arrested a man they described as the head of Boko Haram in Chad and north Cameroon. Baana Fanaye, alias Mahamat Moustapha, was responsible for logistics, recruitment and arms trafficking, Khamiss said.
"Arms were recovered from his house as well as two cases containing documents and a letter from Abubakar Shekau to Boko Haram cadres in Africa," Khamiss said.
Shekau is the militant group's overall leader.
Chad has played a leading role in helping Nigerian forces win back territory from Boko Haram, which has mounted a six-year insurgency to carve out an Islamist caliphate in Nigeria's northeast and attacked neighbours Niger and Cameroon.
Its capital is a command centre for a regional force in the fight grouping troops from Nigeria, Chad, Cameroon and Benin as well as a base for a 3,000-strong French operation fighting militancy in the region.
Despite territorial setbacks in recent months, Boko Haram has shown its ability to strike back in the Lake Chad region in a series of attacks in Nigeria, Niger and Chad.
Chadian security forces on Sunday arrested 60 suspected militants and dismantled the cell responsible for the June 15 attacks, which they said were conducted by militants from Chad, Cameroon, Mali and Nigeria.