At least 23 people have been killed and more than 100 injured in suicide attacks in the Chadian capital N'Djamena, officials say.
Attackers on motorcycles blew themselves up outside two police buildings, say witnesses.
Chad's government blamed Nigeria's militant Islamist group Boko Haram, but the group has not commented.
It has carried out frequent suicide bomb attacks during its six-year insurgency in neighbouring Nigeria.
There was a distressing scene outside the police headquarters where one of the bombs went off, reports the BBC's Will Ross from Lagos.
Corpses and body parts lay on the sandy street outside the police headquarters, alongside the wreckage of motorbikes used by the suicide bombers, he says.
Boko Haram had "made a mistake targeting Chad", and would be "neutralised", said Communications Minister Hassan Sylla Bakari, quoted by Reuters, adding that all four of the attackers had been killed.
He condemned the "appalling and barbaric attack,'' saying that it "would not diminish Chad's determination and commitment to fighting terrorism,'' AP news agency reports.
Chadian forces have played a key role in helping Nigeria battle the jihadist group, and the headquarters of a regional force is being set up in N'Djamena.
The group has never targeted N'Djamena before, but this attack should not come as a huge surprise given Chad's role in fighting the insurgency, our correspondent says.
Large numbers of troops have been deployed on the streets of the capital and a ban has been imposed on cars with tinted windows.
Boko Haram gunmen have previously been active around Lake Chad close to the Nigerian border.
Video statements from the Islamist group have previously criticised and taunted Chadian President Idriss Deby.
The Nigerian army has begun moving its headquarters from the capital Abuja to Maiduguri, the capital of the north-eastern Borno state at the heart of the insurgency.
Boko Haram has lost most of the territory it had controlled following the regional offensive.
But it has continued to stage deadly attacks in Nigeria.