Two drivers - Ed Hensley and Ryan Grundy - were taken to hospital with a possible rib fracture and concussion, said Century Downs track superintendent Don Monkman.

"Neither of them wanted to go to the hospital, but that's typical of horsemen," said Monkman.

A third was uninjured.

Standardbred racing, also known as harness racing, returned to Calgary with the opening of Century Downs racetrack in April 2015. (Associated Press)

One horse was euthanized, according to Martin Ratz, head of security at Century Downs.

Standardbred Canada said five-year-old pacer Pass The Port had to be put down due to a knee injury.

The gelding was a seven-time career winner with over $85,000 in earnings for owner, breeder and trainer Sam Johnson, said Standardbred Canada's official Trot Insider news feed.

"[It] looked like the horse was tired ... and made a break," said Monkman, sparking the chain of events that saw riders tossed around and horses downed.

"It happens... It's an unfortunate thing," said Monkman.

This is the second crash at Century Downs, but the first time a horse has been euthanized, Ratz told CBC News.

Another horse suffered a cut to its leg, but is expected to recover, he added.

The accident comes as Century Downs marks its one-year anniversary of opening in April 2015.

Horse Racing Alberta, the sport's governing body, will investigate the crash.

The races were delayed by about 15 to 20 minutes as a result of the crash. No patrons were involved.

A crowd watches the races at Century Downs, near Cross Iron Mills, on Sunday prior to the crash. (Century Downs)

With files from Kate Adach, Julien Lecacheur and Caroline Wagner