Twenty people are dead and 26 have been injured in a mass shooting at a Walmart store in El Paso, Texas.

"On a day that would have been a normal day for someone to leisurely go shopping turned into one of the most deadly days in the history of Texas," Governor Greg Abbott said, announcing the death toll of 20.

Twenty-six people, including a four-month-old baby, a two-year-old child and a nine-year-old, were wounded at the shooting in the Cielo Vista mall, El Paso Police Chief Greg Allen said.

One suspect, a 21-year-old male, is in custody. He has been identified as Patrick Crusius from Dallas.

Disturbing surveillance footage allegedly shows Crusius, who was wearing "miltary grade" earmuffs, walking through the front entrance of the Walmart store with an AK-47 assault rifle.

Crusius allegedly went on a 20-minute shooting spree at the Walmart store, with one witness describing how "he went from aisle to aisle of the store, shooting with rage".

One law enforcement official told the Washington Examiner he "shot and killed locals that were fundraising outside the Walmart selling water. Children and adults".

After a SWAT team swooped, Crusius surrendered calmly to police and was led away from the scene.

Shocked bystanders snapped images of the alleged gunman walking with police in handcuffs.

Texas Congresswoman Veronica Escobar has described the scene as a "massacre".

Talking to CNN, she said, "I don't want to announce anything yet but I will say the numbers are shocking."

There were initial reports of multiple shooters but Sgt Robert Gomez later said "we have ruled out multiple shooters."

Reports on social media indicate that a manifesto written by the alleged shooter was circulated on various websites but this has not been confirmed.

The document allegedly claimed a "Hispanic invasion of Texas" was the motive for the massacre.

Police were called to the Cielo Vista Mall just after 10am local time (1am AEST).

Witnesses said a shooter opened fire with a "rifle" or some other "semi-automatic" type weapon, KTSM 9 News reported.

Other witnesses have described the shooter carrying an AK-47.

US President Donald Trump responded to the incident on Twitter. "Terrible shootings in El Paso, Texas. Reports are very bad, many killed....God be with you all!"

Eleven of the victims have been transported to the University Medical Centre of El Paso, according to Ryan Mielke, the hospital's Director of Public Affairs. Their injuries range from severe to non-life threatening.

Del Sol Medical Centre has also received 11 people from the scene of the shooting, according to hospital spokesman Victor Guerrero.

A woman named Vanessa told Fox News she was walking into Walmart when she heard what she describes as "fireworks, really loud fireworks."

"You could hear the pops, one right after another and at that point as I was turning, I saw a lady, seemed she was coming out of Walmart, headed to her car," she said. "She had her groceries in her cart and I saw her just fall."

Walmart released a statement saying the company was "in shock" over the incident.

Witness Ray Holgin told CBS News that he was about to go into the Walmart when he heard at least 10 gunshots.

Mr Holgin said he thought he saw at least two people with guns.

He told CBS News that he saw at least one person, an elderly lady, fall to the ground, but he's not sure if she was shot.

The mall complex is near Interstate 10 and on El Paso's east side.

At least three other businesses in the area are also on lockdown, including a Red Lobster franchise and a Hooter's location, CNN is reporting.

HERO SAVED KIDS AS SHOTS RANG OUT

A hero of the Walmart shooting has told how he tried to pick up and carry as many terrified children as he could and run to safety.

Military man Glen Oakley saw terrified people cowering in a Foot Locker store and children running without their parents, scared and alone as the shots rang out.

He told MSNBC that he pulled his own gun out as he heard the first gunshots but quickly his attention turned to the children and as he ran he picked up as many as he could and carried them to safety.

Mr Oakley told MSNBC: "I see a whole bunch of kids running around without their parents, I got my bag in my hand I'm trying to pick up as many as I can and run out

"They're so anxious, they're dropping out of my hand.

"I made my way out and when I got out, I guess one of the cops thought I was the shooter so I had to show them I have a licence to carry.

"I called a friend, and I'm here now, but it was just a whole bunch of kids. I'm shaking. A whole bunch of kids. They were without their parents and stuff. I tried to pick up as many as I could and bring them out with me."

Mr Oakley said he feared the police thought he was the shooter and he had to explain that he had a licence to carry a firearm.