BBC News reports that President Abdul Fattah al-Sisi has declared a three-day period of national mourning over the attack, and vowed to hunt down and punish the perpetrators.
"Vicious terrorism is being waged against the country's Copts and Muslims. Egypt will emerge stronger and more united from this situation," al-Sisi said.
The explosion on Sunday, caused by a device containing at least 26 pounds of TNT, was carried out inside the chapel adjoining the main hall of the church, officials said.
Mena Adel, a church member, told International Christian Concern, a persecution watchdog group, "The church bombing was planned and carried out meticulously. The explosion took place in the pews of the women."
"I think that someone seized the opportunity of overcrowding Copts who were entering the church and was able to enter the church among them and put the bomb under the church pews and then went out," Adel added.
Cathedral worker Attiya Mahrous told The Associated Press, "I found bodies, many of them women, lying on the pews, it was a horrible scene."
Reuters reports that although no group has yet claimed responsibility for the attack, supporters of the Islamic State terror group celebrated the deaths of the victims on social media.
"God bless the person who did this blessed act," one IS supporter wrote on Telegram.
The terror group, which is fighting to keep hold of its captured territory in Iraq and Syria, has committed extreme abuses against Coptic Christians, and in February 2015 released a video showing the mass beheading of 21 Copts in a recording titled "A Message Signed With Blood to the Nation of the Cross."
IS has carried out various other attacks on Copts, who make up only 10 percent of the Egyptian population, and in July it was reported that an IS branch in the Sinai Peninsula shot and killed 46-year-old Coptic Priest Raphael Moussa in the town of El-Arish.
World powers, including the U.N., the U.S., and the Vatican, have all condemned Sunday's attack, with the Obama administration vowing to "continue to work with its partners to defeat such terrorist acts."
Pope Francis also spoke out against what he called the continuation of "brutal terrorist attacks," and said he was praying for the dead and wounded.
William Stark, ICC's Regional Manager, said that the latest attack was both shocking and devastating.
"We mourn for the families that have been destroyed by this bombing and condemn the people or groups responsible for it. This must come as a wakeup call to the Egyptian government and the international community that the Christian population in Egypt is in grave danger from religious attacks," Stark said.
"Entire Christian communities have been assaulted by mobs of Muslim radicals on four separate occasions in 2016 because there was a rumor that a church was being constructed. Now, we have witnessed one of the worst assaults on Egypt's Christian community in years. More must be done to protect Christians and their places of worship in Egypt," he urged.