Two police officers have been killed when a "large number of gunmen" attacked a Nigerian police base in the capital, Abuja, enabling five suspected robbers to escape, police say.
A police statement said the attack was repelled but that 30 detainees escaped, 25 of whom were recaptured.
Suspected robbers and militants from the Boko Haram Islamist group are often held at the police base.
No group has said it carried out the attack
Boko Haram wants to establish Islamic law in Nigeria and has killed hundreds of people this year, mostly in the mainly Muslim north.
The attack on the Special Anti-Robbery Squad (Sars) headquarters in Abuja comes a day after a double suicide bombing killed 11 people in a church inside a military barracks in Kaduna state, north of the capital.
BBC Nigeria correspondent Will Ross says the high-level security breaches are embarrassing for the government.
Many Nigerians are still shocked by Sunday's bombing, wondering how the attackers could have entered the barracks, which houses Nigeria's elite military training centre, without some inside help, he adds.
The Sars base is near several government buildings and security is normally extremely tight in the area.
One Abuja resident said he heard gunfire for about half an hour in the early hours of Monday morning.
The suspects escaped during the confusion of the attack, the police statement said.
None of those who fled were held on terror-related charges, while two of the attackers were arrested, it said.
It also stressed that no explosive devices were used during the attack.
An AFP journalist at the Sars headquarters says there are no visible signs of damage to the building, which is heavily guarded with two armoured vehicles outside.
It is where suspects are held when they are first transferred to the capital.
While most Boko Haram attacks are carried out in the north, it has previously targeted Abuja.
Last year, a suicide bomber from the group attacked the main police headquarters in the capital killing six people.
The militants have also previously attacked prisons and freed hundreds of suspected Boko Haram members.