The tragic death of two horses at the Calgary Stampede Friday night have animal activists calling for a halt to the popular event.

"Just say no to that part of the alleged entertainment where animals are put at risk night after night," Fiona Shedden, from the Calgary Animal Rights Coalition, told CTV Calgary.

The two deaths occurred during the sixth heat of the Rangeland Derby.

As Ray Mitsuing's drove his chuck wagon team down the backstretch, his left lead horse suffered a heart attack.

When the horse collapsed on the track, it caused a number of the other horses to fall down.

The horse directly behind the lead horse in the hitch broke its leg from the fall.

Veterinarians arrived on the scene shortly after the incident and decided to euthanize both horses.

The Calgary Humane Society investigated the chuck wagon deaths.

"It's an unfortunate incident, and that the appropriate measures were taken to ensure that it was addressed," said the society's Joan Treich.

For Mistuing, who was not injured, this is not the first time he has lost a horse in a race.

Four years ago, another horse belonging to the chuck wagon driver also collapsed and died.

Equine heart attacks among chuck wagon horses are not that unusual, according to one former chuck wagon driver.

"I've had probably three or four in 24 years with heart attacks," said Jim Nevada.

This is not the first such animal tragedy to strike the Stampede.

Last year, animal activists were outraged when nine rodeo horses died, falling off a bridge into the Bow River during the Stampede celebrations.

The mishap occurred when some of the 200 participating horses got spooked, and the animals rushed on to the bridge.

A subsequent investigation was unable to determine what caused the horses to panic and fall to their deaths.

Four years ago, seven animals were killed at the rodeo. Of those, six were horses injured during the chuck wagon races and one was a calf that suffered a broken leg during the calf-roping event.

In 1986, 12 animals, including six horses, were killed during a chuck wagon competition at the Stampede.

This year's 10-day event is expected to attract more than one million visitors.

With a report by CTV Calgary's Kevin Rich