Suspected Islamist suicide bombers killed at least 27 people in the Chadian capital N'Djamena on Monday in what appeared to be retaliation by Nigerian militant group Boko Haram for Chad's leading role in a regional offensive against it.
At least 100 people were injured in two simultaneous attacks at around 9:00 a.m. (0800 GMT) on a police headquarters and training school. It was the first strike of its kind in the central African nation, which has emerged as a firm Western ally against Islamist groups in the Sahel.
The government, which said that four Boko Haram fighters were among the 27 dead, announced a raft of measures to tighten security in the capital which serves as the headquarters for a 3,000-strong French mission - known as Barkhane - fighting militancy in the region.
Chad, whose oil revenues have helped it emerge as a military heavyweight in the region, has been a driving force behind a campaign that has inflicted a series of defeats on Boko Haram since January.
"Boko Haram is making a mistake by targeting Chad," Communications Minister Hassan Sylla Bakari said on state television, noting that two bombers were involved in each attack. "These lawless terrorists will be chased out and neutralized wherever they are."
Most of the dead and wounded were found in the police training school where a suicide bomber entered the compound and detonated his charge among a group of cadets, authorities said.
A second suicide bomber blew himself up outside the gates of the school, killing several people.
One witness at the central police station, where a third suicide bomber on a motorbike struck, told Reuters by telephone that he had seen three bodies on the ground.
Photos circulated on Twitter of several blood-stained bodies and damaged motorbikes reportedly used in the attack.
"We could see dead bodies and wounded everywhere on the ground," said a witness, who identified himself as Yerima, who rushed from his office after hearing the explosion. "There were five or six dead bodies and many wounded."
The French foreign ministry condemned the incidents and said it would stand by Chad in the fight against Islamist militancy.
Wounded from the attack were taken to three hospitals, which were overwhelmed by the influx of patients. In the Liberty hospital, a Reuters cameraman witnessed wounded members of the security forces splayed across the bloodstained, sandy tiled floor as doctors scrambled to treat them.
Chad has lost dozens of soldiers fighting al-Qaida-linked groups in northern Mali and Boko Haram in northern Nigeria. The first known attack by Boko Haram on Chadian soil took place in February on the shores of Lake Chad and has been followed by a handful of other isolated incidents.
However, despite threats by Boko Haram leader Abubakar Shekau to strike at Chad in retaliation, N'Djamena had escaped attack so far.
The riverside capital on Cameroon's border is the headquarters for a regional taskforce grouping troops from Nigeria, Chad, Niger, Cameroon and Benin to fight Boko Haram.
Despite losing control of a swathe of territory in northeastern Nigeria this year, Boko Haram has carried out a spate of bombings in recent weeks in the area that has killed about 100 people.
"If this is confirmed as Boko Haram, it sends a bit of a signal," Antony Goldman, a business consultant and long-time Nigeria watcher, said.
"They have carried out attacks in Chad, Niger and Cameroon before but it is an escalation to strike N'Djamena and highlights that they still have the strength to hit sensitive targets," he said.