Militants set off bombs at the Sufi mosque in the Sinai Peninsula during Friday prayers. They then fired on worshippers.
The attack, which is reported to have been conducted by an Islamic State affiliate, was the largest single targeting of Egyptian civilians since the military's 2013 overthrow of the president.
Three days of mourning have been declared while security is being increased across the country.
Writing on Twitter the Archbishop of Canterbury described the incident as terrible beyond words.
He added he was praying for peace and justice.
The attack on a mosque in #Sinai, #Egypt is terrible beyond words. We pray for all affected, for the Egyptian Government, and for peace and justice.
— Justin Welby ن (@JustinWelby) November 24, 2017
Theresa May has labelled the attack as sickening while US President Donald Trump said it was "horrible and cowardly".