Dramatic footage shows UFC legend Stephan Bonnar being pinned to the ground and cuffed as he was arrested for DUI.

The fighter, nicknamed “The American Psycho,” was detained Sunday afternoon in Las Vegas after motorists reported his “erratic” driving at 90 miles per hour, police said.

Witness Katie Dyer, 29 said: “If nobody had stopped him I think he would be dead.

“He was going to hit something, whether it was a car or a wall, and another person could have died.”

Dyer was part of a convoy moving south along I-15 into Las Vegas when she says she saw Bonnar’s red Cadillac dart across the midsection of the road.

Footage filmed by her 10-year-old son, Chance, shows Bonnar crawling along, straddling the emergency lane and holding up traffic.

Dyer claims that she, along with other members of the Jeep club JeepFam, went on to “box in” Bonnar and reduce their speed, forcing him to stop.

Passing truckers, meanwhile, arranged their vehicles in a line across the road, which brought the convoy to a standstill at around 2 p.m.

Police and traffic cops were called to the scene, and footage shows a Nevada highway patrol officer arresting Bonnar.

Video shows Bonnar, 41, looking flushed as he is held face-down to the gravel by four of the Jeep club members.

The front bumper of his red car is smashed up and a yellow luggage tie — used by the motorists to detain him until police arrived — hangs from the driver-side door.

Bonnar retired from the UFC in 2012 but made a brief return to the sport in 2014, when he fought fellow legend Tito Ortiz in Bellator.

However, Bonnar is best known for being half of the most important fight in UFC history when he lost to Forrest Griffin in the finale of the first edition of “The Ultimate Fighter” reality show in 2005. The fight, a three-round war, is widely credited with being the spark that lit the fire that ultimately led MMA to being the sport it is today.

Dyer, a pediatric nurse, said: “We were coming on the I-15 southbound into Las Vegas and he came across the dirt, coming into the south. He didn’t stop or wait or anything, he came straight out into the traffic and he was very lucky.”

“We have radios so we said, ‘Who is calling the police?’ One of our people caught up to him and he seemed to be laying down. He wasn’t sitting up in the driver’s seat. He was going from shoulder to shoulder, from the emergency lane all the way across. Two truckers had slowed down to stop him.

“We carried on and boxed our Jeeps around him. There were six of us. We slowly stopped with the box around him.”

She says once they came to a stop, she and the other motorists raced around to the driver’s side to pull him out of the car and take his key.

“By the time we got over there, his eyes weren’t open, and he wasn’t moving.

“We were trying to find his keys but he wakes up.”

“We were all panicking. He had the car in drive and was hitting the car behind and mine in front. He was ramming into my Jeep.

“We literally had to tie his arms to his door.

“We have ratchet straps that we would use for luggage, so we tied his right arm to his right door frame and his left arm to his left door frame.”

She said law enforcement arrived around 20 minutes later. Medics performed an assessment and he was taken into custody.

Bonnar was arrested on suspicion of driving under the influence, unsafe starting or moving of a stopped vehicle and resisting a public officer.

Bonnar remains in custody following the incident, according to Las Vegas Metropolitan Police Department records.

Bail was set at $22,000.

Dyer added: “We definitely thought people were dying when we saw him. A lot of families were traveling back because it was the weekend, and his driving was erratic.”

Trooper Jason Buratczuk of the Nevada Highway Patrol said: “Witnesses were reporting speeds of over 90 miles per hour and unsafe lane changes.

“When the trooper arrived on scene he observed the driver of the Cadillac had been physically restrained by citizens and appeared to be incoherent.”

He said Bonnar was cuffed and restrained. The smell of alcohol was “emitting” from the driver and he displayed “signs of intoxication.”

“Medical staff responded to the scene and determined nothing medically was abnormal with the driver,” he added.