A bombing at a chapel adjacent to Egypt's main Coptic Christian cathedral killed 25 people and wounded another 49 during Sunday Mass, one of the deadliest attacks carried out against the religious minority in recent memory and a grim reminder of Egypt's difficult struggle to restore security and stability after nearly six years of turmoil.

The attack came two days after a bomb elsewhere in Cairo killed six policemen, an assault claimed by a shadowy group that authorities say is linked to the outlawed Muslim Brotherhood. That group — called "Hasm," or "Decisiveness" — distanced itself from the attack in a statement issued Sunday night, saying it does not as a principle kill women, children, the elderly or worshippers.

The statement, at least in theory, leaves the extremist Islamic State group or like-minded independent militants as the chief suspects.

There was no immediate claim of responsibility for Sunday's attack. However, Islamic militants have targeted Christians in the past, including a New Year's Day bombing at a church in the Mediterranean city of Alexandria in 2011 that killed at least 21 people. More recently, churches and Christian property in southern Egypt were targeted in the aftermath of the military's July 2013 ouster of an Islamist president. Those were blamed on Brotherhood supporters and ultraorthodox Salafi Muslims.

The Islamic State group has targeted Christians in the Sinai Peninsula, where it primarily wages attacks against security forces. However IS attacks on the Egyptian mainland have largely been confined to security personnel and judicial officials.

Regardless of who is behind the bombing, the attack was likely to deal a setback to Egypt's struggle to regain normalcy and revive its ailing economy since a popular uprising toppled autocrat Hosni Mubarak's regime in 2011. Moreover, the attacks this past week were almost certain to undermine the modest recovery made in recent months by the vital tourism sector.

Egypt's official MENA news agency said an assailant lobbed a bomb into a chapel adjacent to St Mark's Cathedral, seat of Egypt's Orthodox Christian church and home to the office of its spiritual leader, Pope Tawadros II, who cut short a visit to Greece to return to Cairo after the blast.

Egyptian state TV and the Health Ministry put the casualty toll at 25 dead and 49 wounded.

Witnesses said the explosion may have been caused by an explosive device planted inside the chapel. A senior church cleric, Bishop Moussa, said at the cathedral that there were unconfirmed reports that a woman posing as a worshipper left a bag in the chapel's women's section before slipping out. Conflicting accounts are common in the immediate aftermath of attacks.

The blast took place as a Sunday Mass in the chapel was about to end and coincided with a national holiday in Egypt marking the birth of Islam's Prophet Muhammad. Most of the victims are thought to be women and children.

State television aired calls by several Cairo hospitals treating the wounded for blood donations and President Abdel-Fattah el-Sissi declared a three-day state of mourning.

"The pain felt by Egyptians now will not go to waste, but will instead result in an uncompromising decisiveness to hunt down and bring to trial whoever helped — through inciting, facilitating, participating or executing — in this heinous crime," a presidential statement quoted the Egyptian leader as saying.

An Associated Press reporter who arrived at the scene shortly after the blast saw blood-stained pews and shards of glass scattered across the chapel's floor. Men and women wailed and cried outside. AP photos showed a broken pair of ladies' spectacles on the ground next to a girl's boots with leopard spots and a pink ribbon.

"I found bodies, many of them women, lying on the pews. It was a horrible scene," said cathedral worker Attiya Mahrous, who rushed to the chapel after he heard the blast. His clothes and hands were stained with blood and his hair matted with dust.

"I saw a headless woman being carried away," Mariam Shenouda said as she pounded her chest in grief. "Everyone was in a state of shock. We were scooping up people's flesh off the floor," she said.

"There were children. What have they done to deserve this? I wish I had died with them instead of seeing these scenes."

The attack has drawn a flurry of condemnations by government and religious leaders as well as assertions of unity between Egypt's Muslim majority and Christians, who account for about 10 percent of the country's 92 million people.

An angry crowd of several hundred people gathered outside the cathedral, chanting anti-government slogans and calling for the sacking of the interior minister, who is in charge of security. "We sacrifice our soul and blood for the cross," some of them chanted. Others sang religious hymns and, raising their hands, screamed "Lord, have mercy."

Scuffles broke out with the police when the protesters tried to push police lines to move closer to the cathedral, but there were no immediate reports of arrests. Police in full riot gear later arrived at the scene.

Egypt has seen a wave of attacks by Islamic militants since the military in 2013 overthrew President Mohammed Morsi, a freely elected leader and a senior Muslim Brotherhood official. Many of Morsi's supporters blamed Christians for supporting the overthrow, and scores of churches and other Christian-owned properties in southern Egypt were ransacked that year.

The authorities have since 2013 waged a sweeping crackdown, jailing thousands of mostly Islamist dissidents and killing hundreds in clashes sparked by demonstrations.

Egypt's Christians have long complained of discrimination in Egypt, contending they are denied top jobs in a wide range of fields, including academia and security apparatuses. Many Christians were relieved when Morsi and his Muslim Brotherhood were ousted in 2013 after just one year in power.

The church and many Christians have since rallied behind el-Sissi, although there have been growing voices of dissent within the community, arguing that little has changed in their lives under his rule — with authorities failing to halt frequent attacks on their churches and property in rural provinces south of Cairo.