One of the official pacers at the London Marathon has said she and fellow runners were treated "horrifically" during the race.
Liz Ayres was asked to run the course in 7.5 hours to aid participants.
She said runners were called "fat" and "slow" by contractors and volunteer marshals - and one woman received chemical burns from the clean-up operation that began around them.
Marathon organisers said they were "very sorry to hear" of her experience.
Like many other marathons, London asks volunteers to run at specific paces during the race as a timing aid for those participating.
This was the first year the London Marathon had recruited people to run at paces slower than six hours.
Ms Ayres told the BBC's Victoria Derbyshire programme that organisers had intended to make the run "more inclusive", with about 200 runners finishing the course at the same time or later than her.
But, she said, despite running at the requested speed, the clean-up operation had begun around her and other runners and they had been "told to hurry up".
She added that abuse had also been directed towards them by official marathon representatives, such as cleaning contractors and marshals.
This included comments such as: "If you weren't so fat, you could run," and: "This is a race, not a walk."
Ms Ayres said she would "rather the race was cancelled than people being spoken to like that".
"I had runners that were crying - ones saying they were going to go home and quit," she said.
Those affected had been running for charities.
Some had been slower due to injuries, or not having been able to train due to family circumstances, Ms Ayres added.
Ms Ayres said similar issues had also been reported by other pacers ahead of her.
"The 6.5-hour pacer said she experienced this, too," she said.
"If you look at the timings of people who finished, that means about 1,000 people were affected.
"That's almost one in every 40 runners."
Ms Ayres said runners on Tower Bridge had also had to "dodge round sewage collection lorries" and run through chemical spray used to clean the streets.
One woman, Sarah Benjafield-Clarke, told the Victoria Derbyshire programme that her GP had confirmed that a blister she had developed from running during the race had developed into a chemical burn.
Ms Ayres also said that as early as the three-mile mark, water stations had been packed away and she had called the London Marathon team only to be told she was lying and that the water stations were still open.
James Miller, 35, had been running for a dementia charity.
He finished in just over eight hours and told the BBC it was "really demotivating to see the course being dismantled around us".
"The worst part was the clocks and timing mats being taking away, so when I passed the 30 and 35km points my time wasn't recorded and I wasn't able to keep track of the progress I was making towards the finish line.
"I even had to ask for directions at a couple of points as the route wasn't obvious.
"It was like you were forgotten about."
London Marathon event director Hugh Brasher said: "We work hard to provide the best possible experience for every runner in the London Marathon and we were very sorry to hear about the experience of Elizabeth and a small number of other runners on Sunday.
"A senior member of our team called Elizabeth yesterday to find out more and we are now looking into this in detail as part of a full investigation.
"We'll be talking to the people involved to find out what happened and we'll also be contacting the runners who were in the group being paced by Elizabeth."
This year's marathon was completed by a record 42,549 runners.