The security forces had sealed off the area around the presidential palace, as well as the police headquarters - which was one of the bombers' targets.
Vehicles with tinted windows had been barred from the streets and cars travelling near mosques, churches and markets were routinely searched, AFP journalists reported.
Chad, which has taken a lead role this year in a regional offensive against the Islamist militants operating out of neighbouring Nigeria, had previously come under attack from Boko Haram in border areas.
But Monday's attacks, which targeted the police headquarters and a police academy, were the first in the capital, where they caused deep shock.
"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.
President Idriss Deby's 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, both Chad and 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.
According to a statement released Tuesday, Hollande spoke with Deby by telephone and hailed the Chadian leader's "brave" fight against terrorism while assuring him of France's support.
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 also 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" and encourage people to "monitor the movements of suspicious people".
President Mahamadou Issoufou of neighbouring Niger firmly condemned the "acts of unspeakable cruelty" in a statement read Monday on state television.
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.
Months before that decision, troops from Chad and Niger began a ground and air offensive on Nigerian soil and took back big swaths of territory from the Islamists, whose name loosely translates as "Western education is forbidden".
The movement is believed still to be holding more than 200 schoolgirls abducted in a raid on a state school at Chibok in northeastern Borno State in April 2014.
UN Secretary General Ban Ki-moon, in a statement, condemned Monday's attacks in N'Djamena and praised Chad "for its courageous role in the fight against Boko Haram".
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