Four runners collapsed and died yesterday during the 13-mile Great North Run.
The causes of death were not disclosed but organisers said the weather was very warm and many runners had suffered heat-related problems.
More than 50,000 people took part in the half-marathon from Newcastle to South Shields, which was celebrating its 25th anniversary.
Paul Bouch, 25, a student from Newcastle who completed the run, said: "After about 10 miles, the ambulances were everywhere and some of the people looked in a bad way. But it was difficult to get the ambulances to the roadside because it's just so packed with people.
"This year, it was just so much harder. I'm just dehydrated and have got a headache more than anything. There's plenty of water along the way but you just try to shut the pain out and carry on."
A spokesman for the organisers said: "It was an extremely warm day, but it started off pretty cold and became a lot warmer, which can, unfortunately, led to people suffering problems.
"We have extensive medical facilities at the course that have been developed over the years, some of the best around."
A North East Ambulance Service spokesman said: "We dealt with a higher number of cases this year compared to last year's run. We don't have any specific figures yet, but there was a particularly busy two-hour period around lunchtime.
"The only thing we can put this down to is the warm conditions. To have four fatalities in the one run is unusual. All four were taken to South Tyneside General Hospital."
A spokesman for Northumbria Police said: "Four male participants have died and all the relatives have yet to be told so no further details will be released at this time."
The men's race was won by the Eritrean Zersenay Tadesse, 23, who set a new world record for the half-marathon, completing the course in 59 minutes and five seconds, beating the previous record set in Madrid in 2000 by one second. Tadesse said: "I didn't expect to break the record. I was very surprised."
John Mayock , 34, was the first Briton to complete the race, finishing sixth in one hour, three minutes, 27 seconds.
Derartu Tulu, from Ethiopia, an Olympic 10,000 metres gold medallist, won the women's race in one hour, seven minutes, 33 seconds.
The event -the brainchild of the former athlete Brendan Foster, who was inspired by the Round the Bays race in Auckland, New Zealand, in 1979 - is expected to raise £10m for charity.