A powerful bomb has ripped through a police headquarters in Egypt's Nile Delta region, killing at least 15 people and wounding 130 others.
Officials say at least 12 of the blast victims in the city of Mansoura were police, and suspect the weapon was a car bomb.
The attack is one of the deadliest against Egyptian security forces since the army deposed Islamist President Mohamed Morsi in July.
The interim government suggested the Muslim Brotherhood was behind the attack, calling the group a terrorist organization. Prime Minister Hazem el-Beblawi vowed that those responsible "will not escape justice."
The Brotherhood issued a statement condemning the attack. It also accused the prime minister of using "inflammatory statements designed to create further violence, chaos and instability."
In Washington, the U.S. State Department issued a statement condemning the attack "in the strongest possible terms" and extending condolences to the families of the victims.
In Mansoura, witnesses said rampaging crowds torched buildings and shops thought to belong to Brotherhood supporters. Others burned an empty tourist bus, saying they earlier witnessed one of its passengers flashing a pro-Islamist hand gesture.
Egyptian Information Minister Mohamed Ibrahim visited the blast site and said the attack was an attempt to affect the country's constitutional referendum set to take place next month.
"Just yesterday we arrested four activists who have confessed to some of these incidents, and they are now aware that we are taking measures against them. And all of these incidents are an attempt to create a diversion and to terrorize people because of the referendum. But I want to reassure people entirely that there is a plan in place in cooperation with the armed forces to protect all of the election centers at the highest level. And God willing the day will pass peacefully."
The Muslim Brotherhood has been the subject of a crackdown since Egypt's military ousted Mr. Morsi in July.
Security forces have arrested much of the Brotherhood's top leadership, including Mr. Morsi, and clashed with those demonstrating against the interim government. More than 1,000 people, mostly Islamists, have been killed in the fighting.
Islamist militants have been blamed for a number of attacks against Egyptian security forces in the past few months, particularly in the Sinai region east of where Tuesday's blast took place.