(CNN)Breaking news update, 10:23 a.m. ET:
A 12-kilogram TNT bomb caused the explosion at a Coptic church in Cairo on Sunday morning, Egyptian state news agency MENA reported, citing security sources. The blast killed at least 25 people and left 31 injured.
An explosion near a key Coptic cathedral in the Egyptian capital has left at least 25 people dead and 31 others injured, local media reported.
The blast occurred in Cairo's Abbassyia district early Sunday morning, according to the government-sponsored Al-Ahram news outlet, citing the health ministry.
"Security forces are searching and screening the area," Al-Ahram's website reported.
The explosion took place in the small church of St. Peter and St. Paul attached to the St. Mark's Coptic Orthodox Cathedral, said the Coptic Church's official spokesperson, Rev. Paul Halim, according to Al-Ahram.
The attack targeted one of the most symbolic religious sites for Copts, an ethno-religious group centered in Egypt. The Coptic Orthodox Church released a statement on its Facebook page, saying in part: "As we are bereaved by this violence and terrorism that attacks worshipers, we pray for these martyrs and for the wounded. The Egyptian church stresses on persevering national unity that keeps all Egyptians on Egypt's blessed land."
Swift condemnationsPresident Abdel Fattah El Sisi condemned the attack and declared a three-day period of national mourning. The Grand Mufti, the highest official of religious law in Egypt, condemned the "deplorable terrorist attack" on the cathedral.
"Attacking churches whether by demolition, bombing, killing those inside, or terrifying ... secure people are prohibited in Islamic Sharia," Sheikh Shawky Ibrahim Abdel-Karim Allam said.
He called for unity against "black terrorism that tries to instigate sectarianism and sedition among the two wings of Egypt -- Muslims and Christians -- in a bid to weaken the nation."
The Grand Mufti also extended his condolences to Pope Tawadros ll of the Coptic Orthodox Church of Alexandria.
Demonstrators gathered outside the cathedral Sunday to voice their anger at police and some pro-government television anchors, according to Al-Ahram. They called for the resignation of the interior minister, Magdy Abdel Gaffer.
Protesters chanted: "This time won't pass, even if we all die. ... This is not sectarian strife, let's get the world to hear."
Sunday's explosion came just two days after two bombs killed six police officers and a civilian in Giza's Haram district, on the street leading to the city's famed pyramids.