Many Britons are anxiously waiting to hear if any of their family or friends have been killed or injured in the deadly stampede in Saudi Arabia.
More than 700 people have been crushed to death near Mecca during the annual Hajj pilgrimage.
It is not known if any of the dead are British, but the Foreign and Commonwealth Office (FCO) is in contact with the local authorities.
Around 20,000 Britons are understood to be making the annual pilgrimage this year, according to Council of British Hajjis Director Rashid Mogradia.
Dilly Hussein, who has relatives in Mecca for the Hajj, told Sky News: "They were so close to the incident but it managed to not affect them.
"My best friend from London actually said that he had been due to join that very group at the Mina but he realised that he was in the wrong tour group so he diverted and joined the correct group.
"Had he joined the wrong group and not realised he was in the wrong group, he could have been one of those 700 people who passed away."
Sahri Jaffri, from Manchester, wrote on Twitter: "News of deaths during the Hajj pilgrimage stampede is making me worry as my mom and her best friend are there now.
"Please pray for their safety and for the others carrying out their pilgrimage too, if you can. It'd mean a lot to me."
A statement put out by the Saudi ambassador to London HRH Prince Mohammed bin Nawaf Al Saud, said: "Our thoughts and prayers are with the victims and families of the tragic accident that occurred at Mina today during Hajj.
"The Custodian of the Two Holy Mosques King Salman has visited the site of the accident and a full investigation into the cause of the tragedy is already under way.
"The Kingdom of Saudi Arabia has directed all the resources at its disposal to care for the victims, and complete the investigation.
"It is still unclear if any British nationals are among the deceased or injured. The Saudi Arabian competent authorities will provide further information as the investigations develop."
The Bradford Council for Mosques is setting up an emergency hotline for relatives of the hundreds of people from the city at the Hajj.
Daud Salaman, chairman of the South Wales Islamic Centre, told Sky News: "It's happened a few times and somebody needs to be held to account for people's safety when they travel there."
Prime Minister David Cameron tweeted: "My thoughts and prayers are with the families of those killed at the Hajj pilgrimage."
A Foreign Office spokesman said: "We are in contact with local authorities and urgently seeking more information following reports of a crush during the Hajj pilgrimage at Mecca."
:: The FCO says worried relatives should call the British Embassy in Saudi Arabia on + 966 11 4819 100 or contact the FCO via Twitter @FCOtravel