A bomb explosion inside the main Coptic Christian cathedral in Egypt's capital, Cairo, has killed 25 people and wounded dozens.

Some reports say a bomb was lobbed into a chapel adjacent to one of the cathedral's walls in the centre of the city but others said it was planted inside.

The spiritual leader of Egypt's Orthodox christian community, Pope Tawadros II has his office in St Mark's Cathedral.

State television reported that a security official said a bomb made of TNT appeared to have been the cause of the explosion.

Ambulances lined up outside the church in the capital's Abbasiya district to evacuate the dead and wounded.

A crowd gathered outside the church chanting: "Tell the sheikh, tell the priest, Egyptians' blood is not cheap".

There was no claim of responsibility for the attack, but jihadists in Sinai have targeted Christians before, as well as Muslims they accuse of working with the government.

Cathedral worker Attiya Mahrous who rushed to the chapel after he heard the blast, said: "I found bodies, many of them women, lying on the pews. It was a horrible scene."

Another eyewitness Mariam Shenouda said: "Everyone was in a state of shock. There were children. What have they done to deserve this? I wish I had died with them instead of seeing these scenes."

The head of Al-Azhar, Egypt's top Sunni authority, condemned the bomb blast as "a great crime" against all Egyptians.

Al-Azhar's grand imam, Sheikh Ahmed al-Tayeb, said in a statement: "The vile terrorist explosion" was "a great crime against all Egyptians".

In April 2013, two people died in clashes outside St Mark's Cathedral.

Copts, who make up about 10% of Egypt's population of 90 million, have faced persecution and discrimination dating back to the 30-year rule of Hosni Mubarak, who was toppled by a popular uprising in 2011.

Dozens have been killed in recent years in sectarian attacks and clashes throughout Egypt.

The deadly attack on Sunday is the second to hit Cairo in two days.

Six policemen were killed on Friday in a bomb attack claimed by a group suspected by authorities of having links to the outlawed Muslim Brotherhood.

Minister Charlie Flanagan unreservedly condemned the bombing which occurred earlier today in Cairo in the Chapel of St Peter and Paul, attached to St. Mark’s Coptic cathedral in Abbassyia district.

In a statement, the Minister said: “I am particularly shocked that the attack took place during a service and was deliberately aimed at worshippers.

"My sympathies are with the people of Egypt at this time, and the families of those who died and were wounded. I also condemn the terrorist attacks which resulted in the deaths of a number of policemen in Cairo over the weekend.

The Irish government condemns all terrorism, and stands in solidarity with the government and people of Egypt at this difficult time."