Police taped off the Third Street S.W. station for their investigation. (Peter Akman/CBC)

A boy police say was no older than five has died after falling from a C-Train platform in downtown Calgary and being hit by a train.

The incident happened just after noon on Tuesday at the Third Street S.W. platform in front of TD Square.

Witnesses told CBC News that an older man was running with a boy to catch the eastbound Somerset C-Train, which was stopped at the platform.

The boy fell head-first as the train began moving and was pulled into the space between one of the C-Train cars and the platform, said Calgary police acting duty Insp. Terry Larson.

Witnesses were distraught as they spoke to police investigators. (CBC)

The child died at the scene.

"Depending on the movement of the train itself, sometimes you can fall in between the train and then get caught in the train as it's moving," explained Brian Whitelaw, Calgary Transit's co-ordinator of public safety and enforcement.

Witnesses screamed and pushed the emergency help buttons inside the train as well as on the platform. Some people tried to administer CPR to the child, said Larson.

Witnesses were visibly distraught as they gave statements to police.

David Ezeronye, 16, had just got off the train when he said he heard screaming.

Emergency crews cover the side of a C-Train with tarp. (CBC)

"We get off only to see a little boy, his head was stuck between the train and the platform, so I'm guessing he fell over," Ezeronye said.

Orange and yellow tarp covered the child's body and the side of the C-Train.

Police have brought in an interpreter to interview the man who was with the boy, who police say is his grandfather.

Calgary Transit will provide security video to police and conduct a mechanical investigation, said Whitelaw.

Traffic in the area was halted for the police investigation.

With files from the CBC's Peter Akman and Brooks Decillia