Police say at least 58 people have been killed and more than 500 injured after a man opened fire at a country music festival in Las Vegas.

The massacre is the worst mass shooting in modern American history, with officials launching an urgent appeal for blood donations.

Gunman Stephen Paddock, 64, shot at people from the 32nd floor of the Mandalay Bay Hotel, diagonally across the street from the concert venue.

Image: SWAT teams stormed Paddock's room and found him dead

SWAT teams broke into the room and found him dead, with police saying they believe he killed himself.

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A "person of interest" Marilou Danley, 62, described as his "companion" by police, has also been tracked down outside the US.

Image: Police tracked down Marilou Danley outside the US

Police have spoken to her and currently do not believe she is involved.

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At least 10 rifles were found in Paddock's hotel room, where he had been staying for four days, but he is not thought to have been part of any terror group.

Moment gunshots ring out at concert

Image: Witness say it was 'chaos' as people ran for their lives

Witnesses described chaos as people ran for their lives as bullets rained down at the outdoor show, next to the city's iconic Strip.

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Concertgoers said they saw flashes from the upper floors of the Mandalay Bay as long bursts of gunfire rang out while singer Jason Aldean performed.

Witness: 'My friend's daughter was shot'

Witness Jack Friedman told Sky News he saw the carnage unfold from the Delano hotel bar, which overlooks the concert ground.

"We saw bodies hitting the ground. We saw people getting shot," he said.

"It was so well lit up because of all the lights of the concert, you could clearly see dozens and dozens of bodies on the ground, just piled up on the ground.

"Watching people run, and bodies hitting the floor as people were running."

Image: The killer opened fire on music fans from the Mandalay Bay Hotel

Image: Police officers point their weapons at a car driving down closed Tropicana Ave after an attack at the Route 91 Harvest country music festival in Las Vegas Image: Officers stop the man who drove down Tropicana Ave near Mandalay Bay Casino Image: A man in a wheelchair is helped away Image: Las Vegas police patrol along the streets outside the the Route 91 Harvest country music festival grounds Image: Police and rescue personnel gather at the intersection of Las Vegas Boulevard and Tropicana Ave Image: People tend to the wounded outside the festival ground. Continue through for more pictures /

The massacre happened as more than 22,000 people attended the last day of the three-day Route 91 Harvest Festival.

Two off-duty police officers, including one from the Las Vegas department, are among the dozens of victims.

'They kept shooting and shooting'

President Trump, addressing the nation from the White House, called the attack an "act of pure evil".

My warmest condolences and sympathies to the victims and families of the terrible Las Vegas shooting. God bless you! — Donald J. Trump (@realDonaldTrump) October 2, 2017

A witness, Brianna, speaking to Las Vegas station KSNV claimed a man and a woman had been ejected from the concert because the woman was telling people they were "all going to die".

"She had been messing with the lady in front of her, telling her she was going to die, that we're all going to die," she said.

"They escorted her out to make her stop messing around with all the other people in front of them."

The witness described them as Hispanic and looking like "everyday people".

Witness: She said we were all going to die

Jason Aldean posted on Instagram: "Tonight has been beyond horrific. I still dont know what to say but wanted to let everyone know that Me and my Crew are safe.

"My Thoughts and prayers go out to everyone involved tonight. It hurts my heart that this would happen to anyone who was just coming out to enjoy what should have been a fun night."