MANSOURA, Egypt - A powerful blast ripped through a police headquarters in an Egyptian Nile Delta city Tuesday while top security officials met to work out arrangements for a forthcoming constitutional referendum, killing 15 people and wounding more than 100 in the deadliest bombing yet in a campaign of violence blamed on Islamic militants.

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 antiterrorism 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 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 members.

Egypt has seen an escalating campaign of bombings, drive-by shootings, assassinations and mass killings, mainly against security forces, in the aftermath of Morsi's ouster and the subsequent crackdown on the Brotherhood. Thousands of Morsi supporters have been arrested - the latest, Hesham Kandil, the prime minister during Morsi's presidency, who was reported arrested Tuesday to serve a previously issued one-year prison sentence.