MANSOURA, Egypt — A powerful blast ripped through a police headquarters in an Egyptian Nile Delta city Tuesday, killing 15 people and wounding more than 100 in the deadliest bombing yet in a campaign of violence blamed on Islamist militants.
The attack, which occurred as top security officials met to work out arrangements for an upcoming constitutional referendum, underlined the vulnerability of Egypt’s police amid fears of increased militant violence before 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 military chief General 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.
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.
Thousands of Morsi supporters have been arrested. Hesham Kandil, the prime minister during Morsi’s presidency, was reportedly arrested Tuesday to serve a previously issued one-year prison sentence.
Most attacks have been centered on 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, which has emerged as the main Sinai-based militant group.
The group has claimed most of the biggest attacks of past months, including a failed attempt to assassinate the interior minister in Cairo in September. Only days earlier, the group threatened more attacks, saying it considers Egyptian troops and police to be infidels because they answer to the secular-leaning military-backed government.
The bombing at the security headquarters in Mansoura — a provincial capital 70 miles north of Cairo that is considered a stronghold for the Brotherhood — was the most significant yet outside the Sinai. The same building had been targeted in July, when an explosive planted outside killed a policeman and wounded another.
Tuesday’s 1:10 a.m. blast brought down an entire section and side wall of the five-floor building. Dozens of parked cars and police vehicles were incinerated, and several nearby buildings were damaged, including a bank and theater. Windows of nearby houses were shattered.
Two top officers were among the dead, and Mansoura’s security chief was injured, losing an eye, the state news agency MENA reported.