Police had bolstered security around the country’s churches for days, especially at the cathedral east of Cairo where President Abdel Fattah al-Sisi came to pay his respects to the ancient community.
He walked in alongside Coptic Pope Tawadros II and took to the altar amid ululations and cries of “we love you”.
“We love you too,” he responded. “You are our family, you are from us, we are one and no one will divide us.”
Sisi said the cathedral, in a new administrative capital Egypt is building, was a “message to the world, a message of peace and a message of love”.
Military and political officials also attended the mass, conducted in a mix of ancient Greek and Coptic.
“It really is excellent… I love him,” said church volunteer Sandy Atef.
“I love him because he saved us from the (Muslim) Brotherhood,” said Hedy Elwahsh, an Evangelical Christian who attended the mass.
She was referring to the 2013 overthrow of Islamist president Mohamed Morsi by the military, led by Sisi.
Since then, security forces have sought to quell attacks led by the Egypt branch of the Islamic State group, which has increasingly targeted Christians.
Tightened security around the cathedral and other churches were testament to fears of a another attack.
Police set up barriers around the church and spread out on the street leading to it while congregants took pictures before a Christmas tree outside.
Within the church, some congregants had small Egyptian flags and used cellphones to take pictures of public figures as they entered.
IS jihadists have taken aim at other civilians, including more than 300 Muslim worshippers massacred at a mosque last November.
But they have focused on the ancient Coptic community, the largest Christian group in the Middle East.
In December 2016, an IS suicide bomber killed almost 30 worshippers at a church in Cairo located in the Saint Mark’s Cathedral complex, the seat of the Coptic papacy.
In the Sinai Peninsula, where IS is based, hundreds of Christians were then forced to flee after a wave of assassinations.
IS jihadists killed more than 40 people in twin church bombings in April and a month later shot dead almost 30 Christians as they headed to a monastery.
The year ended with an IS jihadist killing nine people in an attack on a church in a south Cairo suburb.
Copts, who make up about 10 percent of Egypt’s 96 million people, have long complained of discrimination and intermittent sectarian attacks.