Egyptian security forces gather at the site of an explosion at the Saint Mark's Coptic Orthodox Cathedral in the capital Cairo. Image Credit: AFP

Cairo: At least 25 people were killed on Sunday after a bomb exploded inside a church next to Cairo’s main Coptic Cathedral, marking the deadliest attack in years against a Christian site in the mostly Muslim country.

Forty-nine others were injured in the explosion which occurred early Sunday when dozens of people were attending a mass service inside the church, the Health Ministry said.

The blast was caused by a device packed with around 12 kilograms of the highly explosive TNT substance, security sources said.

No one claimed responsibility for the bombing.

In the aftermath, security forces deployed in large numbers around the St Mark’s Coptic Orthodox Cathedral located in the central Cairo quarter of Abassia. 

Dozens of angry Christians showed up nearby, chanting slogans against the interior minister. The protesters claimed inadequate security measures allowed the attack to take place.

“We sacrifice our souls and blood for you, the cross,” they shouted. “Either we bring back their right or die like them,” they added, referring to the victims.

Riot police stationed in the place prevented young protesters from removing barricades outside the cathedral to enter it.

“This is terrible. I can’t believe that any human being who has a heart will try to kill people while they are praying to God,” said Mounir Boutros, a 52-year-old Christian man.

Grief-stricken women stood by consoling each other.

President Abdul Fattah Al Sissi denounced the “terrorist act” and vowed revenge.

“The pain felt by Egyptians at this moment will not go futile,” Al Sissi said in a statement issued by his office.

“It will result in unshakable determination to track down and put on trial anyone involved in inciting, facilitating or participating in this dastardly act.”

He announced three days of mourning over the attack.

The country’s chief prosecutor meanwhile ordered an inquiry by the State Security Prosecution into the blast.

The attack came a day after Egypt’s highest appeals court confirmed a death sentence against a leading extremist charged with murdering 25 Egyptian policemen in Sinai in 2013.

Workers at the cathedral, inaugurated in 1968, said the bomb that caused Sunday’s explosion had likely been placed under a seat in the church’s women section.

Parts of the church were damaged and its glass windows shattered because of the powerful explosion.

Nearby hospitals appealed for blood donation for the injured.

The attack prompted Coptic Pope Tawadros II to cut short a visit he was making to Greece and return to Egypt, according to a Coptic official.

Egypt’s prestigious Islamic centre of learning, Al Azhar, condemned the attack.

“Targeting places of worship and killing innocent people is a criminal act that violates teachings of Islam,” Al Azhar said in a statement.

It expressed “full solidarity” with the Coptic Church “known for its patriotic stances”.

In 2013, Pope Tawdros II supported in public the army’s ouster of Islamist president Mohammad Mursi.

Several churches were attacked by Islamists in the violence that hit Egypt after Mursi's overthrow.

Al Sissi, who as a defence minister led Mursi’s toppling, is hugely popular with Egypt’s Christian minority.

Since he took office in 2014, Al Sissi visited the St Mark’s Cathedral on the eve of the Coptic Christmas. His visits were unprecedented by an Egyptian president before him.

Christians account for around 10 per cent of Egypt’s mainly Muslim population of 92 million.

Christians have long complained about attacks by militants. Twenty-three people were killed in a bomb explosion on the Two Saints Church in the Egyptian coastal city of Alexandria in January 2011.