At least 28 fans and one driver have been injured by flying debris from a multi-car crash at the Daytona speedway near the end of the nationwide NASCAR race.
Race officials said 14 fans were sent to nearby hospitals, with another 14 being treated at the Florida speedway, which will host the prestigious Daytona 500 race on Sunday.
Joie Chitwood, president of the Daytona International Speedway, said Sunday's main race would go ahead despite the incident.
"First and foremost, our thoughts and prayers are with our race fans," Chitwood said.
"Following the incident, we responded appropriately according to our safety protocols and had emergency medical personnel at the incident immediately.
"We transported 14 people off property and 14 were treated at our on-track care centre. We are in the process of repairing the facility, and we will be ready to go racing tomorrow," he added.
Debris from the 10-car crash flew into the spectator area and amid chaotic scenes, some fans were taken away on stretchers.
The crash sent driver Kyle Larson's car airborne, although he climbed out of the wreckage afterward unhurt.
The wreck, which occurred almost as Tony Stewart was taking the chequered flag for victory, began when Regan Smith turned sideways and a dozen cars bunched behind him.
"My fault," admitted Smith. "I threw a block."
Larson, making his first start in NASCAR's second-tier series, was launched into the catch-fencing.
Larson's car tore a hole in the fence separating the track from the stands. His engine sheared off with at least one tyre and other debris flying into the grandstand.
"I looked in the mirror and that's the worst image I've ever seen in a race in my life," Stewart said.
" We've always known since racing started this is a dangerous sport. "
- Regan Smith, driver
"I saw a tyre about 10 feet from me, just a row above me with a man under it and people yelling for help," he said in a telephone interview.A fan who identified himself only as "Tyler" sent amateur video to ESPN and spoke with the sports network about the frightening scene.
"Our prayers and thoughts are with everybody they are working on," NASCAR President Mike Helton said of those being treated by emergency medical personnel.
None of the drivers involved in the crash was injured, but driver Michael Annett was hospitalised with chest bruising after hitting a safety barrier in an earlier crash in the race.
"We've always known since racing started this is a dangerous sport," a subdued Stewart said. "As much as we want to celebrate, I'm more concerned about the fans and the drivers right now."
The race came on the eve of the season-opening event in NASCAR's top-flight Sprint Cup series, the Daytona 500.
Both O'Donnel and Chitwood said they expected Sunday's race to start on schedule.