N'Djamena -Chad's President Idriss Deby vowed on Tuesday there would be no impunity for those who masterminded twin suicide bombings that killed 24 people and wounded more than 100 day earlier.
The bombings, the first such attacks in the capital N'Djamena, have been blamed on Boko Haram jihadists who have previously attacked villages along Chad's border with Nigeria.
"Whoever is responsible (for the attacks), ... will answer for their action. And I promise that this action will not stay unpunished," said Deby, who had just returned from an African Union summit in Johannesburg.
"This invisible hand and its co-sponsors have hit a peaceful people who were only seeking peace for themselves and for Africa and for the world," he added.
Deby said he was "not surprised" by the attack given Chad's lead role in a regional offensive against Boko Haram fighters operating out of northeastern Nigeria.
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Security was stepped up in N'Djamena on Tuesday, with scores of police and soldiers patrolling the streets and stopping cars for security checks.
Vehicles with tinted windows had been barred from the streets and the area around the presidential palace and the police headquarters -- which was one of the bombers' targets, along with a police academy - had been sealed off.
Monday's attacks on the police caused deep shock in the capital.
"It's terrible...I never would have thought that such a thing would happen in N'Djamena," Ali Gamane, an engineer working for the agriculture ministry, said.
Doctors at the city's Amitie hospital were struggling to cope with the influx of wounded.
"Many of the injured risk dying if the public doesn't come forward to donate blood," nurse Ache Zenaba warned.
Four "terrorists" were also killed in the blasts, according to the authorities, who gave no further details.
The government called for calm.
"These attacks, which aimed to strike fear into the people, will not slacken Chad's determination to combat terrorism," the government said, assuring the situation was "entirely under control."
Although Boko Haram has yet to claim responsibility for the bombings, France, which relies heavily on N'Djamena in the fight against jihadist groups in the Sahel region, accused the militants of being behind the attack.
"There is no doubt that Boko Haram is responsible and will be brought to justice for this new humanitarian horror," French President Francois Hollande said during a visit to Algeria.
Hollande spoke with Deby by telephone and hailed the Chadian leader's "brave" fight against terrorism while assuring him of France's support, a statement from the French president's office said.
UN Secretary General Ban Ki-moon also condemned Monday's attacks and praised Chad "for its courageous role in the fight against Boko Haram".
Boko Haram's leader Abubakar Shekau had threatened several times to attack Chad and other countries that joined forces against the group, whose bloody six-year insurgency is increasingly spilling across Nigeria's borders.
In February, the group carried out its first attack inside Chad, crossing Lake Chad by boat under cover of darkness to attack the village of Ngouboua, torching homes and killing several people.
Monday's attacks were the first however in the capital of the former French colony, which hosts the headquarters of France's Sahel counter-terrorism force, Operation Barkhane.
"We're used to seeing these things (terrorist attacks) in other places but thought it would never happen here," said Andre Toal, a civil servant, told AFP, admitting to "living in fear".
Opposition politician Brice Mbaimon called on the government to "quickly implement a plan of national vigilance".
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President Mahamadou Issoufou of neighbouring Niger condemned the attacks as "acts of unspeakable cruelty".
Issoufou urged the international community to back member states of the Lake Chad Basin Commission in a joint struggle against Boko Haram, which killed 74 people, including 28 civilians, in a raid inside Niger on April 25.
Nigeria, Chad, Niger, Benin and Cameroon agreed last week to set up a regional task force of 8,700 soldiers, police officers and civilians, based in N'Djamena, to combat Boko Haram.
Earlier this year, Chad and Niger launched a joint ground and air offensive on Nigerian soil against the insurgents, wresting back some territory from the extremists.
Boko Haram, which launched a bloody campaign for an Islamic caliphate in northeast Nigeria six years ago, is believed to be still holding more than 200 schoolgirls abducted in a raid on a school at Chibok in northeastern Borno State a year ago.