The incident took place at the Bridge Bay Campground, south of Fishing Bridge, according to professional photographer Mike MacLeod from Bozeman, Montana, who witnessed the attack and spoke to Cowboy State Daily.

MacLeod said the man was walking with his grandson when the agitated bison suddenly charged at them.

“I was just trying to get some dramatic footage of that bison having a fit,” MacLeod told Cowboy State Daily. “It’s changed my idea of what to expect from these guys at this time of year, because I would not have predicted that happening.”

The National Park Service has not released any official details about the incident.

A video shared on social media showed the tourist at the Bridge Bay Campground as his wife pointed out the bison while they were entering the area. The man was seen taking photographs of the animal from what appeared to be a safe distance.

MacLeod said the bison later moved through the campground and charged towards a group of children who were taking pictures from a distance.

“He started walking through the campground,” MacLeod said. “He was coming up to this group of kids, who were taking pictures on their cellphones from a good distance away, and then the buffalo charged these kids.”

“They weren't even in that camping loop. They were walking along the road, quite a ways away from the bison, and it started running at them,” he added.

According to reports, the tourist’s grandson managed to escape, but the grandfather was struck by the animal.

Describing the attack, MacLeod said the bison caught the man with its left horn near his hip and threw him into the air.

“He made a perfect flip and landed on his side. The bison was at least six feet tall, and (the victim) was several feet above him,” he said.

Emergency medical personnel from Yellowstone National Park arrived at the scene shortly afterwards and provided medical assistance.

MacLeod said he later spoke with the victim’s grandson, who informed him that his grandfather had suffered “some pretty significant injuries” and was still in serious condition.

Authorities have repeatedly warned visitors to Yellowstone to maintain a safe distance from wild animals, including bison, which can be unpredictable and extremely dangerous.