President Abdel Fattah al-Sisi inaugurated Egypt's largest church and mosque in the New Administrative Capital on Sunday, the eve of Coptic Christmas, in a message of tolerance in the predominantly Muslim country.

Copts, the largest Christian minority in the Middle East, held a a midnight mass in the Cathedral of the Nativity, billed by the government as the Middle East's largest church, a few hours after the inauguration, which President Sisi also attended.

Coptic Christians make up an estimated 10 per cent of Egypt's nearly 100 million people and have long complained of discrimination under laws that favour Muslims.

They have also increasingly been targeted in recent years by Islamist militants including Islamic State, which is waging an insurgency in the north of the remote Sinai Peninsula.

Foreign dignitaries and officials including Palestinian President Mahmoud Abbas and Arab League Chief Ahmed Aboul Gheit flanked Sisi at the opening, state television showed.

Angham, a prominent local singer, sang for Muslim-Christian coexistence as a display of fireworks lit the skies over the two houses of worship.

'This is an important moment in our history,' Sisi said in a speech as he opened the cathedral. 'We are one and we will remain one,' he added, referring to Egyptian Christians and Muslims.

'On this day we see you have fulfilled this promise and here we are witnessing a great opening on this grand occasion,' said Pope Tawadros, the head of the Coptic church who presided over midnight mass later in the evening with Sisi in attendance.

US President Donald Trump also praised the opening of the church and the mosque.

'Excited to see our friends in Egypt opening the biggest Cathedral in the Middle East. President Al-Sisi is moving his country to a more inclusive future,' Trump tweeted.

Video messages of support were played from the region's Christian leaders as well as from Pope Francis. Speaking in Italian, Francis said: 'With joy I greet all of you on the joyful occasion of the dedication of the new Cathedral of the Nativity, built in the new administrative capital. May the Prince of Peace give to Egypt, the Middle East and the whole world the gift of peace and prosperity.'

Adorned with Coptic icons, the cathedral can accommodate more than 8,000 worshippers while the al-Fattah al-Aleem Mosque can hold nearly double the number. Both are located in the new administrative capital, a major development located some 45 km (28 miles) east of Cairo.

Sheikh Ahmed al-Tayeb, Grand Imam of the al-Azhar Mosque, said Islam obliges Muslims to safeguard and defend houses of worship, whether Muslim, Christian or Jewish.

Contractors have been clearing debris from the perimeter of the cathedral in the last two weeks in preparation for its grand opening.

The new Egyptian capital, announced in March 2015, is intended partly to reduce crowding in Cairo but will also be home to government ministries and an airport. The government is expecting to begin moving to the new premises later this year.

The inauguration ceremony took place on the Coptic Church's Christmas Eve. Just a few hours before, a police bomb squad officer was killed trying to defuse an improvised explosive device near a Cairo church; the participants observed a minute's silence in memory of him.

More than 100 Christians have been killed in terrorist attacks during the last two years.

Additional reporting by Reuters.