A gunman opened fire at a supermarket in Boulder, Colorado, on Monday, killing 10 people, including the first police officer to arrive on the scene, before the bloodied suspect was arrested in the second deadly US mass shooting in a week.

Police gave few immediate details of the latest shooting and no known motive for the violence, which unfolded at about 3pm at a King Soopers grocery store in Boulder, a north-central Colorado city at the eastern foot of the Rockies, about 28 miles northwest of Denver.

Frantic shoppers and employees fled for cover through the supermarket as law enforcement officers swarmed the scene, located about two miles from the University of Colorado's flagship campus.

Sarah Moonshadow, 42, a customer and Boulder resident who was in the store with her son, Nicholas, recounted scenes of pandemonium as gunfire rang out from inside the store.

“We were at the checkout, and shots just started going off,” Moonshadow told Reuters.

“And I said, 'Nicholas get down'. And Nicholas ducked. And we just started listening and there, just repetitive shots [...] and I just said, 'Nicholas, run.'”

Moonshadow said she tried to go to the aid of a victim she saw lying on the pavement just outside the store, but her son pulled her away, telling her, 'We have to go.'”

The woman broke down in sobs, adding: "I couldn't help anybody."

The bloodshed came less than a week after gun violence last Tuesday that left eight people dead, including six Asian women, at three day spas in and around Atlanta.

A 21-year-old man has been charged with those killings.

Handcuffed and bleeding

Boulder Police Chief Maris Herold, her voice choking with emotion, said 10 people died in the attack at King Soopers grocery.

Among them was 51-year-old policeman Eric Talley, an 11-year veteran of the Boulder police force, who she said was the first officer to respond to the shooting. He was the father of seven children, according to his father.

Police said the gunman, who was not publicly identified, was injured in the violence but they gave no further details.

Video footage from the scene broadcast earlier by television stations showed a shirtless, bearded man in boxer shorts being led away from the store in handcuffs, before he was placed on a stretcher and loaded into an ambulance.

The man in custody appeared to have a bloody leg and was limping.

Colorado has been the scene of some of the most shocking mass shootings in modern US history.

In 2012, a young man dressed in tactical gear burst into a movie theater in the Denver suburb of Aurora during a midnight screening and sprayed the audience with gunfire, killing 12 and wounding 70.

In 1999, a pair of students went on a shooting rampage at Columbine High School near Littleton, Colorado, murdering 12 classmates and a teacher before taking their own lives.

One man who was shopping inside the King Soopers store with his wife told KCNC-TV that he heard three gunshots and ran through the store with other customers to escape through a loading bay at the rear of the building.

He said the shots sounded like they came from the checkout area at the front of the store.

Authorities gave few details about how the shooting unfolded. Commander Kerry Yamaguchi of the Boulder Police Department told reporters several hours after the fact that investigators were just starting to process the crime scene.

“Boulder has suffered a terrible and horrific mass shooting today,” Boulder County District Attorney Michael Dougherty said at a news conference later in the evening.

Video footage taken by an onlooker and posted to social media shows two bodies lying in the grocery store parking lot, before the person with the camera walks inside and films a third body lying there, as three shots are heard in the background.

The United Food and Commercial Workers International Union, Local 7, which represents 32 employees at the King Soopers outlet, expressed gratitude to emergency responders, customers and staffers who “acted swiftly to prevent even greater loss of life”.

“There are news reports that after gunshots rang out, grocery workers helped customers in the store find safety, directing shoppers to an exit at the back of the store, and assisted one another to escape the danger inside,” the statement said.

The Table Mesa area of Boulder where the shooting occurred is home to a residential neighborhood and the hilltop laboratory of the National Center for Atmospheric Research.

Police declined to say what kind of weapon was used, but the Denver Post reported that initial 911-emergency calls described the suspect's firearms as a “patrol rifle”.