The semi-official Ahram newspaper posted on its website that three bombs had been planted out of which two went off almost at the same time, reported Xinhua.
The third one in a car nearby was defused, the website quoted an anonymous security official as saying.
The attack took place at the Nile Delta Governorate of Daqahliya, official news agency MENA said, adding that a nearby five-store bank building collapsed and dozens of vehicles were wrecked due to the strong explosion.
Most of the victims were police conscripts, Omar el-Shawatqi, governor of Daqahliya, told reporters.
"They have been rushed to nearby hospitals," he said. Mansoura is the capital of the Daqahliya Governorate.
The head of Mansoura's security directorate was among the injured and the death toll is expected to increase, as more people are still trapped inside the building, he added.
Interim Prime Minister Hazem el-Beblawi said the bombing was a terrorist attack and vowed to severely punish the attackers.
On Tuesday, he also declared Muslim Brotherhood, the country's oldest and most influential Islamist movement, as a terrorist group, official news agency MENA reported.
Tensions are running high in Egypt since the fall of former Islamist President Mohamed Morsi in early July, with a string of bombings attacks taking place countrywide.
Much of the Islamist militants based in the restive northern Sinai Peninsula were blamed for the attacks. The military then launched an offensive to crack down on them.
--IANS (Posted on 24-12-2013)