More than 700 people have been crushed to death after a stampede on the outskirts of the holy city of Mecca during the annual hajj pilgrimage.
Officials in Saudi Arabia estimated the number of deaths at 717 while shocked members of the Muslim community are hoping for a clear explanation of what went wrong.
It is not known if any of the dead are British but the Foreign & Commonwealth Office (FCO) has said it is in contact with the local authorities.
Prime Minister David Cameron tweeted: "My thoughts and prayers are with the families of those killed at the Hajj pilgrimage."
Rashid Mogradia, of the Council of British Hajjis, a national organisation which looks after the welfare of pilgrims who are going to Mecca, told BBC News: "Our thoughts and prayers are with all those who have been affected.
"It is tragic news. On the one hand we have Muslims all around the world and here in the UK celebrating Eid while we have this tragic incident unfolding before us in Mina, Saudi Arabia.
"We have been in contact with a number of tour operators who have taken around 20,000 British pilgrims out for the hajj making up that figure of two million pilgrims."
He said: "We are seeing on a hourly basis and now minute-by-minute the numbers are increasing and we hope that will be the end of what we hear.
"There is a lot that has been done as far as infrastructure has been concerned, however we need to understand what has gone on today and how it can be improved so that further casualties and further incidents do not take place."