Sweltering heat made the London Marathon an even bigger test of endurance yesterday as a record number of runners took part.
A 22-year-old man who collapsed after completing the race died today, it was reported.
Race organisers said 36,391 people started the 26-mile course in difficult hot, sunny and windless weather.
Temperatures hit just below 21C (70F) at midday, almost equalling the 1996 record, and rose slightly higher later in the day.
Runners reported "nightmare" conditions as ambulance staff said they dealt with a higher than usual number of patients.
London Ambulance Service said this year's event was "busier" than usual, with participants and spectators affected by the heat.
A spokeswoman said some people were suffering heat-related problems such as dehydration as well as sprains and other injuries.
A St John spokeswoman said it treated 5,032 people of which 73 were taken to hospital.
Celebrity chef Gordon Ramsay, who was running his eighth marathon, said the conditions were "extraordinary".
He said: "It was like running in a desert today. I stopped to help one guy. It was quite bad. They were dropping like flies."
Another runner, Michelle Dewberry, who won the BBC television show The Apprentice, said she saw someone being resuscitated after about four miles.
She said: "I have seen lots of people passed out and being resuscitated. It is scary, scary stuff."
Champion athlete Liz McColgan, who won the marathon in 1996, said some runners were in trouble. She said: "It must be a nightmare for some people. I saw a lot of people walking with cramps and things. I think a lot were suffering."
England rugby World Cup team member Matt Dawson said he saw other runners pass out in front him. He said: "You could see people just burning up in front of you, as I was."
A team of 1,500 St John Ambulance volunteers and doctors cared for runners, with some being taken away in wheelchairs and on stretchers.
Prior to the race, medical director Dr Sanjay Sharma, said runners should take frequent mouthfuls of water during the gruelling race.
A St John Ambulance spokeswoman said there had been reports of a "high" number of people collapsing at the finish line, but she said this was not unusual.
"People find they have got everything taken out of them," she said.
Runners were buoyed by hundreds of thousands of supporters who took advantage of the sunny weather to line the London route.
Ramsay added: "The big advantage of having weather like this today was there were thousands of people.
"They would not let you stop, walk or stretch and I think I have overdosed on jelly babies."
Other celebrity competitors included Olympic 400m hurdler Sally Gunnell, wheelchair basketball player Ade Adepitan and model Nell McAndrew who ran with her mum, Nancy.
Among the professionals, Kenyan Martin Lel won the men's race. The UK's number one Jon Brown withdrew because of sickness.
Chunxiu Zhou, of China, won the women's race as Briton Dave Weir held on to his wheelchair title.
Many runners were raising money for charities including Cancer Research UK, Oxfam, Wellchild and the British Heart Foundation.
One of them, Steve Chalke, 51, broke the world record for the most money raised by an individual marathon runner set by Sir Steve Redgrave last year.
Mr Chalke raised #1.82 million for the charity he founded, the Oasis Trust, which builds schools for disadvantaged young people.