The scene unfolded around 10:30 a.m. at an intersection two blocks from the stadium where, hours later, undefeated Oklahoma State secured a lopsided 58-10 victory over the University of Kansas in the homecoming football game. The driver of a Hyundai Elantra struck an unmanned police motorcycle before running into the crowd, Capt. Kyle Gibbs of the Stillwater Police Department said.

The 2-year-old boy died at O.U. Medical Center in Oklahoma City, where hospital officials said four children and three adults were taken for treatment on Saturday.

Three of those patients had been flown from Stillwater Medical Center. Shyla Eggers, a spokeswoman for the Stillwater hospital, said the facility had received a total of 43 patients from the crash who ranged in age from 1 to 66.

The driver, identified as Adacia Avery Chambers, was arrested on charges of driving under the influence, the police said. Ms. Chambers, 25, of Stillwater, was in jail on Saturday, according to Captain Gibbs. She is not an Oklahoma State student, he said.

A woman who answered the phone at the home of Ms. Chambers’s father, Floyd Chambers, said he was not taking phone calls. “He is very devastated,” she said.

Collision investigators were still working at the crash scene Saturday afternoon and said they had not determined if the act was intentional. The crumpled front of the gray Hyundai could be seen at the intersection. A baby stroller stood a few feet away. Several people dressed in the university’s colors, orange and black, were seated on the ground, being tended to by emergency medical workers. Yellow caution tape and orange traffic cones surrounded the area.

Witnesses described seeing a fast-moving sedan and bodies flying, according to news reports.

“Someone just ran a car full speed through the homecoming parade,” Johnathan Peterson, an alumnus, wrote on Twitter. “Inches from hitting us. Lots of people down. Chaos on Hall of Fame.” Two employees of a tint shop in Stillwater said they had seen Ms. Chambers on Saturday morning. They said she had arrived to her job next door at Freddy’s Frozen Custard and quickly left. Although she appeared upset, they said, she did not appear to be under the influence of alcohol or drugs.

Image Adacia Avery Chambers was arrested on charges of driving under the influence. Credit... Stillwater Police Department, via Associated Press

“She just looked upset as she walked by, like she had been crying,” said one of the workers, Jerry Blair. “She walked a perfect line.”

Lexie Pearce, a spokeswoman for the restaurant chain, declined to discuss Ms. Chambers’s employment. She said in an email that the Stillwater franchise was “cooperating fully with law enforcement officials.”

At the news conference, Mr. Hargis, Oklahoma State’s president, said officials had considered postponing the game against Kansas but ultimately decided against it.

“The decision was made that we’re going to play, and we’re going to remember the victims at the game,” he said. “We’re going to move forward, but we will remember what happened today.”

A moment of silence was held for the victims before kickoff, and an American flag was lowered to half-staff.

It was not the first time tragedy had struck Oklahoma State. In January 2001, 10 people, including two men’s basketball players, were killed in a plane crash returning from a game in Colorado. And in November 2011, the women’s basketball coach and an assistant coach were among four people killed in the crash of a small plane.

“The families, I know, 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’ll do it again. God bless, and we pray for everyone.”

Oklahoma State boasts that it hosts the nation’s largest homecoming gathering, drawing tens of thousands of visitors a year for a weekend of celebration. Stillwater, which sits between Oklahoma City and Tulsa, about 70 miles from each, has a population of about 47,000, but officials say as many as 80,000 descend on the city for homecoming weekend.