What you need to know:
The official Al-Ahram newspaper reported on its website that at least 184people were killed and 80 wounded in the attack, which is unprecedented in a four-year insurgency by Islamist extremist groups.
The Islamic State group's Egypt branch has killed hundreds of policemen and soldiers, and also civilians accused of working with the authorities, in attacks in the north of the Sinai peninsula.
Armed attackers on Friday killed at least 235 worshippers in a bomb and gun assault on a packed mosque in Egypt's restive North Sinai province, in the country's deadliest attack in recent memory.
A bomb explosion ripped through the Rawda mosque frequented by Sufis roughly 40 kilometres west of the North Sinai capital of El-Arish before gunmen opened fire on those gathered for weekly Friday prayers, officials said.
Witnesses said the assailants had surrounded the mosque with all-terrain vehicles then planted a bomb outside.
The gunmen then mowed down the panicked worshippers as they attempted to flee and used the congregants' vehicles they had set alight to block routes to the mosque.
The state prosecutor's office said in a statement that 235 people were killed and 109 wounded in the attack, the scale of which is unprecedented in a four-year insurgency by Islamist extremist groups.
US President Donald Trump condemned on Twitter the "horrible and cowardly terrorist attack on innocent and defenceless worshippers."
A furious Egyptian President Abdel Fattah al-Sisi declared three days of mourning and pledged to "respond with brutal force" to the attack.
"The army and police will avenge our martyrs and return security and stability with force in the coming short period," he added in a televised speech.
UK foreign minister Boris Johnson condemned the "barbaric attack" in a post on Twitter, while his French counterpart Jean-Yves Le Drian expressed his condolences to the families of victims of the "despicable" bloodshed.
Ahmed Abul Gheit, head of the Arab League, which is based in Cairo, condemned the "terrifying crime which again shows that Islam is innocent of those who follow extremist terrorist ideology," his spokesman said in a statement.
IS TARGETING OF SUFIS
There was no immediate claim of responsibility for the attack.
The Islamic State group's Egypt branch has killed hundreds of policemen and soldiers, and also civilians accused of working with the authorities, in attacks in the north of the Sinai Peninsula.
They have also targeted followers of the mystical Sufi branch of Sunni Islam as well as Christians.
The victims of Friday's attack included civilians and conscripts praying at the mosque.
A tribal leader and head of a Bedouin militia that fights IS told AFP that the mosque is known as a place of gathering for Sufis.
The Islamic State group shares the puritan Salafi view of Sufis as heretics for seeking the intercession of saints.
The jihadists had previously kidnapped and beheaded an elderly Sufi leader, accusing him of practising magic which Islam forbids, and abducted Sufi practitioners later released after "repenting."
An IS propaganda outlet had published an interview earlier with the commander of its "morality police" in Sinai who said their "first priority was to combat the manifestations of polytheism including Sufism."
The group has killed more than 100 Christians in church bombings and shootings in Sinai and other parts of Egypt, forcing many to flee the peninsula.
The military has struggled to quell jihadists who pledged allegiance to IS in November 2014.
IS regularly conducts attacks against soldiers and policemen in the peninsula bordering Israel and the Palestinian Gaza Strip, although the frequency and scale of such attacks has diminished over the past year.
The jihadists have since increasingly turned to civilian targets, attacking not only Christians and Sufis but also Bedouin Sinai inhabitants accused of working with the army.
Aside from IS, Egypt also faces a threat from Al-Qaeda-aligned jihadists who operate out of neighbouring Libya.
A group calling itself Ansar al-Islam — Supporters of Islam in Arabic — claimed an October ambush in Egypt's Western Desert that killed at least 16 policemen.
Many of those killed belonged to the interior ministry's secretive National Security Service.
The military later conducted air strikes on the attackers, killing their leader Emad al-Din Abdel Hamid, a most wanted jihadist who was a military officer before joining an Al-Qaeda-affiliated group in Libya's militant stronghold of Derna.