The final race of the Breeders' Cup meeting has been marred by a fatal injury to Mongolian Groom.

Celebrations for the win by Vino Rosso in the Breeders' Cup Classic were overshadowed when Mongolian Groom broke down in the straight with a hind leg injury.

The horse was taken to an equine clinic but euthanised a couple of hours later.

His death is the 37th at Santa Anita since December.

The deaths prompted track owner The Stronach Group to implement changes to rules involving medication and training.

The Breeders' Cup also beefed up its own pre-race exams and observations of runners.

"Everything had been going so great," trainer Bob Baffert said before the death was announced.

"You just don't know when it is going to happen. We try to keep them as safe as we can."

Four veterinarians were consulted before the decision was recommended to euthanise Mongolian Groom.

Cup officials said they have hired Dr Larry Bramlage to conduct an independent evaluation with the results to be made public when completed.

Winning trainer Todd Pletcher said he was concerned about horse safety at Santa Anita coming into the season-ending championships.

"We were anxious, not only for running in huge races like this, but hoping everything would go smoothly and safely," he said.

"Everyone took every precautionary measure they possibly could."

Four horses were scratched from Cup races after pre-race exams by vets found issues concerning enough to keep them in their barns.

Earlier in the day, protesters angered by the 36 deaths stood outside Santa Anita toting signs urging the end of the sport in California.

A short distance away, industry workers feeling pressured by the prospect of losing their jobs rallied to promote racing.