At least two people have been killed after a powerful blast triggered by a suicide bomber struck Nigeria's police headquarters in Abuja, the country's capital.
Boko Haram - a group calling for wider application of Islamic law in Nigeria - has said it was behind the attack.
Abu Zaid, a man claiming to speak for Boko Haram, told the BBC's Hausa language service that his group was responsible.
Olusola Amore, spokesman of the national police, said two people were confirmed dead after a car blew up in the parking lot outside the building on Thursday.
The suicide bomber and a traffic officer, who rode in the loaded vehicle to show the driver where to park, were both killed in the attack, Amore said.
"The body of the suicide bomber has been recovered and a full investigation has commenced," he read from an official statement.
Al Jazeera's Yvonne Ndege, reporting from the blast site, called it "quite a horrific scene" where at least 30 to 40 vehicles were completely burnt and "bodies - pieces of mutilated bodies" were being carried out from charred vehicles.
Umar Mairiga, a spokesman for the Nigerian Red Cross, said that seven injured people were taken to a hospital for treatment and two had already been discharged. Mairiga said it was difficult to confirm how many people had died.
Police said 33 cars had been damaged beyond repair and 40 more had been partially damaged by the explosion, though the building was not affected.
Also on Thursday, another blast at a church in the town of Damboa in the northeast killed at least three children and was also blamed on Boko Haram.
Ndege, our correspondent, added that the attack in northern Nigeria did not come as a surprise.
"Local residents have said that attacks by Boko Haram happen often there," she said.
A series of explosions and attacks targeting police and fire service headquarters had rocked northeastern Nigeria last week, and Boko Haram were assumed to have been behind it.
In a leaflet attributed to Boko Haram and distributed to journalists in the city of Maiduguri on Wednesday, a man claiming to be a Boko Haram spokesman warned that the group would launch more attacks after being angered by comments from the national police chief.
Inspector-general of police Hafiz Ringim said during a visit to Maiduguri on Tuesday that "the days of Boko
Haram are numbered". His office is at the police headquarters in Abuja.
However, the authenticity of the statement could not be verified.
But it did also warn residents of all northern states, including the district where Abuja is located, to stay indoors to avoid getting caught in the violence.
Boko Haram, whose name in the Hausa language means "Western education is a sin,'' has campaigned for the implementation of Sharia law.
The group is responsible for of killings which have targeted security officers, politicians and clerics in Nigeria's restive north over the last year.
Most attacks have occurred in their stronghold of Maiduguri, about 872km from Abuja.
Source: Al Jazeera and agencies