A rugby fan was airlifted to hospital after sustaining "horrific" injuries at the Marriott London Sevens tournament at Twickenham stadium on Saturday.
It is thought the man, in his 20s, fell from a second-storey building on to the concrete steps below at about 6.22pm.
London Ambulance Service (LAS) attended as well as London's Air Ambulance, which transported the man to St Mary's Hospital in London.
A spokesman said: "We sent a number of resources to the scene including an ambulance crew, a fast responder in a car and London's Air Ambulance."
LAS was unable to comment on his condition.
Emma Staples, who attended the tournament, said: "A man, approximately 25 years old, fell from the balcony where people enter and exit the stadium.
"He fell from at least the second storey on to the concrete steps at the bottom.
"I got a look at him and I’m sad to say it looked potentially fatal."
Rugby fans took to Twitter to discuss the "terrible" fall.
Ray Chester tweeted: "Fellow in terrible fall from second floor to concrete steps. Twickenham East Stand. #MarriotSevens".
James Howard wrote: "Anyone know if that guy who fell of the stadium at twickenham sevens survived? Looked horrific."
- Did you see what happened? Contact the news desk by emailing tom.ambrose@london.newsquest.co.uk or calling 020 8722 6330.
Comments: Our rules
We want our comments to be a lively and valuable part of our community - a place where readers can debate and engage with the most important local issues. The ability to comment on our stories is a privilege, not a right, however, and that privilege may be withdrawn if it is abused or misused.
Please report any comments that break our rules.
Read the rules here