The Middle East News Agency said the explosion took place at 1 10 morning (local time), at Daqahliya security headquarters in the Nile Delta province of Mansour, causing collapse of parts of the five-floor building.
A security official says 11 people were killed and 80 injured including the security chief of the city. Most of those killed were among policemen inside the security headquarters whose bodies were buried under the debris. Death toll is expected to rise, the official said.
The attack comes a day after an al-Qaida inspired group called on police and army to desert or face death on hands of its fighters.
Ansar Beit al-Maqdis, or the Champions of Jerusalem, said it considers Egyptian troops to be infidels because they answer to the secular-leaning military-backed government.
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The group and others based in the Sinai have been blamed for a surge of attacks against security forces since a July coup toppled the country's former Islamist president, Mohammed Morsi.
Speaking at a public forum yesterday, military spokesman Ahmed Mohammed Ali said so far the operations have resulted in the killing of 184 militants and the arrest of 803 others. He said about 25 percent of those killed and arrested are foreign fighters, but didn't provide further details.
Ansar Beit al-Maqdis is believed to have ties with Palestinian militants in the neighboring Gaza Strip, and officials have said other foreign militants have found refuge in Sinai during the ongoing turmoil.
Ali also said during the military operations, troops have destroyed about 786 tunnels between Sinai and Gaza, which the government says are used to smuggle militants and weapons.
"We have achieved great successes. We have impacted the main capabilities of the terrorists," Ali told the gathering.