CAIRO -- A powerful explosion tore through a security headquarters in a city about 75 miles northeast of the capital early Tuesday, killing at least 14 people and injuring more than 100 others, state media reports said.
Egyptian authorities described the attack in the Nile Delta city of Mansoura as the most serious of its kind since the military-backed government took power almost six months ago. There was no immediate claim of responsibility, but Egypt's interim prime minister, Hazem Beblawi, called it a terrorist act.
Officials swiftly cast blame on Islamist groups, and called the attack an attempt to derail next month's scheduled referendum on the country's new constitution.
Egyptian television showed floodlit scenes of large crowds gathered outside the partially collapsed security building, while medical personnel worked frantically to treat the injured and searchers began gingerly removing slabs of the wreckage. Twisted metal and broken glass littered the area.
Many people, most of them police officers who were inside the building at the time, were believed trapped in the rubble, and the toll was expected to rise.
In the months since Islamist President Mohamed Morsi was removed from power in a popularly supported army coup, attacks against security forces have become commonplace in the restive Sinai Peninsula, and in and near the Suez Canal city of Ismailia. But they have been far rarer in the capital or other principal cities.
In September, the interior minister escaped an attempt to assassinate him with a suicide bomb in Cairo. A Sinai-based militant group known as Ansar Bayt al-Maqdis claimed responsibility for that attack.
The same group this week threatened more attacks on Egypt's security forces, and urged police and soldiers to desert the ranks.
Mansoura has been the scene of frequent protests by supporters of the Muslim Brotherhood, calling for Morsi's reinstatement.