Thirteen people were killed and over 100 were injured in an explosion at a security compound in the Nile Delta city of Mansoura in Dakahlyia early on Tuesday, security sources said.
The United States mission in Cairo has condemned the blast, saying, "We support the Egyptian government's efforts to bring those responsible to justice."
The Egyptian army said, "These cowardly operations are carried out by a misguided faction that has deviated from the Egyptian national community and adopted violence and treachery."
Egypt's Prime Minister Hazem el-Beblawi released a statement shortly after the attack officially announcing the Muslim Brotherhood as a terrorist organization, but the group itself has condemned the attack.
Attacks on security forces and their offices have noticeably risen since the ouster of the Muslim Brotherhood's Mohamed Mursi from power.
Egypt's army ousted Mursi in July in response to mass demonstrations across the country against his rule, announced early presidential and parliamentary elections and suspended a 2012 constitution written by an Islamist-led assembly.
A new constitution will be voted on in a referendum early next year.