A horrifying crush at the annual hajj pilgrimage to the holy city of Mecca has killed more than 700 people.
The deadliest disaster in 25 years to hit the sacred pilgrimage has killed 717 people and injured 863, according to Saudi Arabia officials, and has left shocked members of the Muslim community calling for a clear explanation of what went wrong.
Some two million people from across the world take part in the five-day pilgrimage which began on Tuesday.
The crush happened in a morning surge of pilgrims at the intersection of streets 204 and 223 in Mina, a large valley about three miles east of Mecca.
The faithful were making their way towards a large structure overlooking three columns where pilgrims carry out a symbolic stoning of the devil by throwing pebbles.
It is not known if any of the dead are British but Foreign secretary Philip Hammond said the Foreign & Commonwealth Office (FCO) is bolstering the size of its team on the ground as it seeks to "urgently gather information about British nationals who may require assistance".
He said: "I was saddened to hear of the enormous loss of life in Mecca. My thoughts are with families and victims at this difficult time. Such a significant tragedy will affect Muslims across the world who take part in the hajj pilgrimage."
FCO staff are "in close contact" with the Saudi authorities and tour operators, and are checking hospitals and other locations.
It is estimated that around 25,000 British nationals head overseas from the UK to Saudi Arabia for the hajj, according to Abta, the travel association.
Each year pilgrims pay hundreds of thousands of pounds to go on the trip which all believers who can afford it are required to perform once, with people spending between £4,000-£5,000 for a typical pilgrimage.
Amateur video and pictures on social media showed images of dead bodies on the ground dressed in the simple terry cloth garments worn during the hajj.
Other pictures show people sitting in wheelchairs and being treated.
Prime Minister David Cameron tweeted: "My thoughts and prayers are with the families of those killed at the Hajj pilgrimage."
A message on behalf of cricketer-turned-politician Imran Khan said he was "deeply saddened" to learn of the stampede.
It added: "May Allah grant the martyrs a place in heaven. Praying for the injured in Mina stampede and for the safe return of all the Hajis."
The tragedy happened as Muslims around the world celebrated the key festival of Eid al-Adha
Mohammed Adree Sharif, who joined pilgrims in the stoning ritual at the hajj, said he was " absolutely shocked" and "horrified" by the deaths as the event had seemed to be well organised.
He told ITV News: "We had heard about things like that happening in the past. The whole point of having different levels and a lot of money being spent by the Saudi government was to make it easier for the pilgrims.
"Today people are supposed to be celebrating Eid as well and yet no one is really celebrating here - we are just wandering around in shock and thinking 'for the grace of God, it could have been us'."
Eid al-Adha is known as the Feast of the Sacrifice as it recalls Abraham's willingness to sacrifice his son in obedience to Allah.
It is the worst tragedy to hit hajj in 25 years and comes just two weeks after a giant construction crane came crashing down on the Grand Mosque in the Saudi city of Mecca, the focal point of the hajj.
A stampede in a tunnel killed over 1,400 pilgrims in 1990. Other fatal incidents have included the death of 244 pilgrims who were crushed in Mina in 2004 while more than 360 pilgrims were killed in a 2006 stampede also at Mina.
Saudi authorities deployed about 100,000 security forces this year to oversee crowd management and ensure pilgrims' safety.
Humza Yousaf, the SNP MSP for Glasgow and the Minister for Europe, tweeted: "My own family is at hajj - big sigh of relief to hear they're safe. Heart goes to those who haven't heard news and to those affected."
Mr Yousaf recalled that the 2007 hajj he attended was very well organised but having spoken to his parents who are there now it seems the current "organisation has not been up to the same standard as it has in previous years - it is quite concerning and quite worrying".
Saudi ministers who have blamed pilgrims for not sticking to the timetables and a lack of discipline are being "wholly unhelpful", according to Mr Yousaf.
He told BBC News: "There should be an investigation in to how this happened and there should be some very serious questions asked about the hajj. It is two million people and (questions should be asked) about whether or not the Saudis have the infrastructure to ensure the safety of those pilgrims."
The Church of England has also offered up prayers for those who have been killed, are working to save lives or are worried about their loved ones.
Zulfi Karim of the Bradford Council for Mosques estimated between 5,000 to 7,000 people from Bradford were at the event.
He said: "We are hoping and praying for those people who are out there. There are a lot of people from the city out there at the moment and we are liaising with the Foreign Office and the tour companies."
The FCO urged worried relatives in the UK to contact the FCO switchboard on 020 7008 1500. Those in Saudi Arabia are asked to contact the British Embassy on: + 966 11 4819 100; or to go online @FCOtravel; or to contact the FCO via Twitter.