An off-duty police officer saved the life of a woman with cancer who was too exhausted from treatment to flee gunfire at the mass shooting in Las Vegas.

Cassidy Huff, who had recently finished a course of chemotherapy and was at the Route 91 festival with her mother, collapsed behind a car when she tried to run.

Officer Mitchell Tosti, a member of the Los Angeles Police Department who was at the festival with his fiancee and friends, spotted Ms Huff and carried her to safety.

"She looked tired, so I grabbed her. She was only wearing one shoe when I saw her," he told NBC.

"I was like, ‘I’ll carry you.’ I told my fiancee to grab the mom and follow us."

They fled until they found another officer who drove Ms Huff and her mother Kelley home.

"I think they're amazing and I think it's amazing that we ran into all off-duty police officers," Ms Huff said.

"I wanted to run but my body wouldn't let me. My legs are still really weak so they buckle under me."

Kelley Huff posted a thank you to Mr Tosti and his fiancee in a Facebook post.

"Just as we crossed the street, Cassidy collapsed behind a car, her frail body could not go anymore," she wrote.

"Before I could even try to think what to do next, an off-duty LAPD officer scooped her up in his arms and said to keep running, we can’t stay here.

Las Vegas shooting – in pictures Show all 15 1 /15 Las Vegas shooting – in pictures Las Vegas shooting – in pictures People scramble for shelter at the Route 91 Harvest country music festival after gun fire was heard Getty Las Vegas shooting – in pictures People carry a person at the Route 91 Harvest country music festival after shots were fired David Becker/Getty Las Vegas shooting – in pictures People run from the Route 91 Harvest country music festival after gun fire was heard David Becker/Getty Las Vegas shooting – in pictures A handout photo released via Twitter by Eiki Hrafnsson (@EirikurH) showing concertgoers running away from the scene (C) after shots range out at the Route 91 Harvest festival on Las Vegas Boulevard EPA/Eiki Hrafnsson Las Vegas shooting – in pictures People lie on the ground at the Route 91 Harvest country music festival after hearing gun fire Getty Las Vegas shooting – in pictures A man in a wheelchair is taken away from the Route 91 Harvest country music festival after hearing gun fire David Becker/Getty Las Vegas shooting – in pictures People stand on the street outside the Mandalay Bay hotel near the scene of the Route 91 Harvest festival on Las Vegas Boulevard EPA/Paul Buck Las Vegas shooting – in pictures FBI agents confer in front of the Tropicana hotel-casino after a mass shooting during a music festival on the Las Vegas Strip Reuters/Las Vegas Sun/Steve Marcus Las Vegas shooting – in pictures Las Vegas police run by a banner on the fence at the Route 91 Harvest country music festival grounds after shots were fired David Becker/Getty Las Vegas shooting – in pictures An injured person is tended to in the intersection of Tropicana Ave. and Las Vegas Boulevard after a mass shooting at a country music festival Ethan Miller/Getty Las Vegas shooting – in pictures Metro Police officers pass by the front of the Tropicana hotel-casino after a mass shooting at a music festival on the Las Vegas Strip Reuters/Las Vegas Sun/Steve Marcus Las Vegas shooting – in pictures A woman sits on a curb at the scene of a shooting outside of a music festival along the Las Vegas Strip AP/John Locher Las Vegas shooting – in pictures A cowboy hat lays in the street after shots were fired near a country music festival in Las Vegas Getty Las Vegas shooting – in pictures Las Vegas Metro Police and medical workers stage in the intersection of Tropicana Avenue and Las Vegas Boulevard South after a mass shooting at a music festival on the Las Vegas Strip Reuters/Las Vegas Sun/Steve Marcus Las Vegas shooting – in pictures Sheriff Joe Lombardo (2-R) speaking during a press briefing in the aftermath of the active shooter incident on Las Vegas Boulevard EPA

"His sweet girlfriend grabbed my hand and pulled me along," she said. "What a selfless thing to do."