Militants have launched a bomb and gun attack on a mosque in Egypt's North Sinai province, killing 235 people, state media say.
The al-Rawda mosque in the town of Bir al-Abed was targeted during Friday prayers.
It is the deadliest attack of its kind since an Islamist insurgency in the peninsula was stepped up in 2013.
Egyptian President Abdul Fattah al-Sisi vowed to respond with "brute force" after talks with security officials.
No group has yet claimed the attack, but militants affiliated with so-called Islamic State (IS) have been responsible for scores of deadly attacks in the province.
They usually target security forces and Christian churches, and the bloody attack on a mosque associated with Sufi Muslims has shocked Egypt.
Witnesses said dozens of gunmen arrived in off-road vehicles and bombed the packed mosque before opening fire on worshippers as they tried to flee.
The assailants are reported to have set parked vehicles on fire in the vicinity to block off access to the mosque.
Pictures from the scene show rows of bloodied victims inside the mosque. At least 100 people were wounded, reports say, overwhelming hospitals.
"They were shooting at people as they left the mosque," a local resident who had relatives there told Reuters news agency. "They were shooting at the ambulances too."
It is the deadliest militant attack in modern Egyptian history. Bir al-Abed is about 130 miles (211km) from Cairo.
Who was targeted?
Locals are quoted as saying that followers of Sufism, a mystical branch of Sunni Islam, regularly gathered at the mosque.
Although Sufis are widely accepted across much of the Muslim world, some jihadist groups, including IS, see them as heretics.
The head of IS's religious police in Sinai said last December that Sufis who did not "repent" would be killed, after the group beheaded two elderly men reported to be Sufi clerics.
The victim of the mosque attack also included military conscripts.
Who might be behind the attack?
Militant Islamists have been waging an insurgency on the Sinai peninsula in recent years, stepping up attacks after Egypt's military overthrew Islamist President Mohammed Morsi after mass anti-government protests in July 2013.
Hundreds of police, soldiers and civilians have been killed since then, mostly in attacks carried out by Sinai Province group, which is affiliated to IS.
Sinai Province has also carried out deadly attacks against Egypt's Coptic Christian minority elsewhere in the country, and said it bombed a Russian plane carrying tourists in Sinai in 2015, killing 224 people on board.
It has been operating mainly in North Sinai, which has been under a state of emergency since October 2014, when 33 security personnel were killed in an attack claimed by the group.
Sinai Province is thought to want to take control of the Sinai Peninsula in order to turn it into an Islamist province run by IS.
An unprecedented attack
By Sally Nabil, BBC News, Cairo
Islamist militants have been operating in northern Sinai for several years, mainly targeting security forces.
But this is the first time that worshippers inside a mosque have been targeted. The number of victims is unprecedented for an attack of this type.
Northern Sinai has been living under a media blackout over the past few years. No media organisations have been allowed to go there, including state-sponsored ones.
The frequency of attacks raises doubts about the effectiveness of military operations. While the army releases statements every now and then, claiming victory in some parts of Sinai, there seems to be no end in sight to the ongoing battle between the army and militants.
What has the reaction been?
President Sisi has declared three days of mourning. The military is reported to have already conducted air strikes in mountains around Bir al-Abed.
"What is happening is an attempt to stop us from our efforts in the fight against terrorism, to destroy our efforts to stop the terrible criminal plan that aims to destroy what is left of our region," Mr Sisi said in a televised address.
Arab League chief Ahmed Aboul Gheit condemned the attack as a "terrifying crime which again shows that Islam is innocent of those who follow extremist terrorist ideology".
In other reaction:
- UK Prime Minister Theresa May tweeted: "Appalled by the sickening attack on a mosque in North Sinai. Condolences with all those in #Egypt affected by this evil and cowardly act."
- US President Donald Trump called the attack "horrible and cowardly".
- French Foreign Minister Jean-Yves Le Drian called the attack "despicable", while Paris Mayor Anne Hidalgo said lights at the Eiffel Tower would turn off to honour the victims
- The city hall in Tel Aviv, Israel, projected the Egyptian flag to show solidarity
Will be calling the President of Egypt in a short while to discuss the tragic terrorist attack, with so much loss of life. We have to get TOUGHER AND SMARTER than ever before, and we will. Need the WALL, need the BAN! God bless the people of Egypt.— Donald J. Trump (@realDonaldTrump) November 24, 2017