“We ended up having to weave through moving vehicles to get through!” she wrote.

Ms. Ayres posted photos showing an obstacle course of maintenance vehicles, and areas where course markers had already been removed. She said her group was told to move out of the way and run on the sidewalk, and water stations had packed up before they reached them. Cleaning crews and race marshals made demeaning comments as they went, she said.

“If you weren’t so fat, you could run,” and “this is a race, not a walk,” were among the comments, Ms. Ayres told the BBC on Thursday. She said she would “rather the race was canceled than people being spoken to like that.”

The director of the London Marathon, Hugh Brasher, apologized to Ms. Ayres and her fellow runners on Thursday. “We are absolutely determined to understand what went on,” he told the BBC. “It will take time to do it, but I can assure you that the investigation will be thorough and as a result of it changes will be made.”

The London race, like marathons around the world, has sought to become more accessible to casual and novice runners, bestowing medals on all finishers and encouraging people to take up exercise and train for the race. It has even collaborated with Guinness World Records to celebrate unconventional achievements at the race. The organization certified 38 records on Sunday, including the fastest marathon in a six-person costume, run in just under six hours.

Event organizers, however, have struggled at times to include the slowest participants, including those who walk most of the course. In the face of much skepticism and blowback from serious athletes, running at the back of the pack has become a global movement, embraced by organizers including in London.