An attack on anti-terrorist police headquarters in Pakistan's largest city, Karachi, has left 20 dead and at least 100 injured.
Police say they exchanged fire with militants trying to storm the Criminal Investigation Department building.
Then a truck laden with explosives slammed into its boundary wall, detonated its load and almost completely destroyed the structure.
The blast could be heard across several miles of the city of 14 million people.
Eyewitnesses said the blast left a crater 12 metres (40ft) wide in front of the gutted building in Pakistan's financial and commercial capital.
TV footage showed bloodied victims being taken away on stretchers and dozens of security officers combing through the wreckage.
"Over a dozen militants tried to storm the building," a police official who was inside the building during the attack told the BBC.
"An exchange of fire took place for at least 15 minutes. We then saw the pick-up truck trying to ram its way inside."
A government spokeswoman, Sharmilla Farooqi, said: "There are five policemen among the dead.
"We have reports that there may be some women police among the casualties because there was a women's police station inside the building."
One witness told the BBC that he had heard the exchange of gunfire before the explosion.
"I was playing tennis across the road at the Karachi Club when I heard gunshots and then a huge blast," said Ali Zaidi.
"Everyone started panicking and running toward the changing rooms. Some of my friends have been injured and have been taken to hospital."
The BBC's Shoaib Hasan, in Islamabad, says that CID officials and their offices - including this building - have been targeted in Karachi in the past.
Our correspondent adds that the latest attack comes a day after the same unit arrested several wanted militants in the city, said to belong to Lashkar-e-Jhangvi - Pakistan's most dangerous militant group.
The group, which is closely linked to al-Qaeda, has been involved in a string of high profile attacks across the country.
It is unclear whether the suspects were being detained in the complex at the time of the attack but the BBC's Aleem Maqbool, in Islamabad, says that in the past, such detainees may well have been held and interrogated in the CID compound.
Mohammad Aslam Khan, of the CID, told the BBC that he believed the arrested men were planning to carry out bombings on Shia processions in the city.
The site of the blast is within a high-security area in the south of the city, known as the "red zone". It is not far from the Sindh province chief minister's residence, the US Consulate and several five-star hotels including the Sheraton.
Other buildings close by were badly damaged in the blast, which shattered windows within a two-mile radius.
The blast took place in the evening rush hour as Pakistan's commercial capital was busy with people leaving work.
No group has immediately claimed responsibility for the attack but the Taliban have been behind a number of similar attacks on police and army compounds in recent years.
The Karachi bombing came a week after a suicide bombing on a mosque packed with worshippers killed 68 people in the north-west of the country. Hours later, a grenade assault on a second mosque in the area killed four people.