Most of the dead from the bombing of a police headquarters in Mansoura early Tuesday were police officers, Middle East Online reported.
Following the blast, Egypt declared the Muslim Brotherhood a terrorist organization.
Omar al-Shawatsi, the governor of Daqahleya, of which Mansoura is the capital, said the blast was from a car bomb.
Sami El-Mihi, the head of security for Daqahleya, was wounded and two of his aides killed, security officials said.
The explosion blew out windows of nearby buildings, the officials said. Its impact could be felt up to 12 miles away.
Following the blast, a spokesman for Prime Minister Hazem Beblawi was quoted by the state-run Middle East News Agency as saying the prime minister "had declared the Muslim Brotherhood a terrorist organization."
The declaration could be another move by the interim government to isolate the brotherhood ahead of a constitutional referendum, Middle East Online said.
The Muslim Brotherhood condemned the Mansoura attack and Beblawi's declaring it a terrorist organization.
"The Muslim Brotherhood considers this act as a direct attack on the unity of the Egyptian people and demands an inquiry forthwith so that the perpetrators of this crime may be brought to justice. It is no surprise that Beblawi, the military junta's puppet prime minister, has decided to exploit the blood of innocent Egyptians through inflammatory statements designed to create further violence, chaos and instability," a brotherhood statement said.
The attack brought condemnation from the Obama administration in Washington.
"The United States condemns in the strongest possible terms today's terrorist attack on the Dakaliya security directorate in Mansoura," State Department spokeswoman Jen Psaki said in a statement.
"We extend our deepest condolences to the families of the victims, and we hope for the quick recovery of the injured.
"The United States stands firmly with the Egyptian people as they work to put their country on the path towards democracy, stability and economic prosperity in an atmosphere free from violence."
Security sources told Ahram Online's Arabic site two bombs exploded simultaneously, one placed in the upper story of the building and another in a car parked outside. The sources said a third bomb was defused.
"This is an act of terrorism that aims at frightening the people and obstructing the road map. The black hands behind this act want to destroy the future of our country," Beblawi told Egyptian satellite channel ONTV. "The state will do its utmost to pursue the criminals who executed, planned and supported that attack."
Since the July ouster of former President Mohamed Morsi, the country has been plagued by bomb attacks, mainly in the Sinai region.