A car has smashed into a crowd during the Oklahoma State University homecoming parade, killing four people, including a two-year-old boy, and injuring 44 in the horror crash that sent some spectators flying through the air "like rag dolls".

Adacia Chambers' Hyundai Elantra struck an unoccupied motorcycle of an officer at the parade, then careered into the crowd. She was taken into custody and police are awaiting the results of blood tests to determine if she was under the influence of drugs or alcohol.

"We treat these like we would any homicide investigation," Captain Kyle Gibbs of Stillwater police said. "It'll probably take several days to get additional information as to the cause of the accident."

Oklahoma University Medical Centre and The Children's Hospital earlier announced that the toddler had become the fourth person to die from injuries from Saturday morning's crash. Five children and three adults remain in hospital.

Chambers' father, Floyd Chambers, of Oologah, told The Oklahoman newspaper he could not believe his 25-year-old daughter, from Stillwater, was involved and insisted she was not an alcoholic.

He described her as "timid" and said she had attended homecoming festivities on Friday night with family members but her boyfriend had told him she was home by 10pm.

"This is just not who she is. They're going to paint her into a horrible person but this is not (her)," Mr Chambers told the paper.

Oklahoma State University president Burns Hargis said there had been a discussion about cancelling the homecoming football game on Saturday afternoon against Kansas, but it was played as scheduled. The victims were remembered with a moment of silence before kick-off, and most of the OSU players knelt on the sideline in prayer.

Even as the game began, some of the bodies remained at the scene of the crash less than three streets away from the stadium. National Guard troops kept watch as Red Cross officials and state medical examiner's office continued their work.

Hundreds of fans wearing the school's bright orange and black colours were forced to walk by the junction as they headed to the game. Some lingered to look at the aftermath - water bottles, blankets, lawn chairs and other items strewn over the road. A grey car with a smashed side and shattered windscreen remained at the scene, as did a crumpled motorcycle.

"I just saw smoke and saw the panic in people's faces as they ran away from the scene," said Geoff Haxton, of Tulsa, who attended the parade with his children.

Another spectator, Konda Walker, from Anchorage, Alaska, told the Stillwater News Press that some people initially thought the crash was part of the show.

"People were flying 30 feet into the air like rag dolls," she told the News Press.

The university posted on Twitter: "Oklahoma State University is saddened by the tragic parade incident earlier this morning. Our thoughts & prayers are with those affected."

It's not the first tragedy to strike events connected to Oklahoma State sports programmes. Ten people, including two OSU men's basketball players, were killed in a 2001 plane crash while returning from a game in Colorado. And Oklahoma State women's basketball coach Kurt Budke and assistant Miranda Serna were among four killed in a plane crash in Arkansas in 2011 while on a recruiting trip.

"The families, I know, and these victims will never be able to understand this, nor will we," Mr Hargis said. "But the Cowboy family pulls together, unfortunately we've had to do it before and we're going to do it again."