MANSOURA, Egypt -- A powerful blast ripped through a police headquarters in an Egyptian Nile Delta city on Tuesday while top security officials met to work out arrangements for an upcoming constitutional referendum, killing 14 people and wounding more than 100 in the deadliest bombing yet in a campaign of violence blamed on Islamists.

The attack underlined the vulnerability of Egypt's police and their weakness in keeping security amid fears of increased militant violence in the lead-up to the Jan. 14-15 referendum.

The vote is a key step in the country's political transition after the military's ouster of Islamist President Mohammed Morsi in July, but it has further stoked political tensions, with Morsi's Muslim Brotherhood protesting against the new charter.

Authorities quickly sought to pin blame for the blast on the Brotherhood, the military-backed interim government's top political nemesis. With the group continuing protests, the government has increasingly depicted it as directly behind the wave of violence, without providing evidence in public.

The attack hikes pressure on the government by anti-Islamists to take tougher action against the group, including enforcing a court-ordered ban on it, possibly declaring it a terrorist organization and passing a controversial harsh new anti-terrorism law. The Brotherhood condemned the attack and accused the government of scapegoating it.

At the funeral for the victims of the blast, including 14 policemen and a civilian, hundreds massed in a main square of the city of Mansoura where the bombing took place, chanting, "The people want to execute the Brotherhood."

They raised posters reading "no to terrorist groups" and pictures of military chief Gen. Abdel-Fattah el-Sissi, who removed Morsi and is the country's most powerful figure. Some in the crowd set fire to the car of a man who flashed a pro-Morsi symbol and others attacked houses of Brotherhood leading member.

Egypt has seen an escalating campaign of spectacular bombings, drive-by shootings, assassinations and mass killings, mainly against security forces, in the aftermath of Morsi's ouster and the subsequent deadly crackdown on the Brotherhood, which left hundreds dead and thousands injured.

Most attacks have been centered in the Sinai Peninsula, where multiple militant groups operate.

, but the insurgency has been spreading to the capital Cairo and heavily populated Delta.

There was no immediate claim of responsibility for Tuesday's blast, but senior military and security officials said it carried the fingerprints of Ansar Beit al-Maqdis, or the Champions of Jerusalem.