Mourners sign crosses, March 23, 2021, placed in honor of the victims, along a fence put up around the parking lot where a mass shooting took place a day earlier at a King Soopers grocery store in Boulder, Colorado. (DAVID ZALUBOWSKI / AP)

A gunman killed at least 10 people, including a police officer, at a supermarket in Colorado on Monday, police said, in the latest shooting to hit a state that was the site of two of the most infamous mass murders in US history.

Police chief Maris Herold said 10 people were killed, including officer Eric Talley, 51, who was first on the scene, paying tribute to his "heroic" colleague

Witnesses at the King Soopers supermarket in Boulder County, 50 kilometers northwest of the state capital Denver, said they initially heard multiple loud bangs outside the shop.

Nevin Sloan, who narrowly escaped with his wife Quinlan, said panic set in as the sound of gunshots drew closer and shoppers were forced to decide whether to stay put or make a break for the exit.

"All of a sudden we started hearing more, like 'bang, bang, bang, bang' and I sprinted over to her and just told her 'hey we need to get out of here'," he told the CBS television network, saying they helped others flee through an emergency exit.

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"I just nearly got killed for getting a soda and a bag of chips," Ryan Borowski, who was in the store when he heard at least eight gunshots, told CNN.

Police chief Maris Herold said 10 people were killed, including officer Eric Talley, 51, who was first on the scene, paying tribute to his "heroic" colleague.

Live-streamed video earlier showed a white middle-aged man－shirtless and seemingly covered in blood－detained by police and led from the supermarket.

A suspect is being held in custody and was the only person suffering "serious injuries at this point", said police commander Kerry Yamaguchi, without confirming the man in the video was the suspect or giving any possible motive.

Dozens of armored vehicles, ambulances and armed personnel including FBI agents and SWAT teams were deployed to the scene.

Herold gave no more information about the shooting, saying the "complex investigation will cost at least five days" to make a conclusion.

'Face of evil'

Colorado governor Jared Polis condemned the "senseless killing".

"Today we saw the face of evil. I am grieving with my community and all Coloradans," he said on social media.

The US western state has previously suffered two of the most infamous mass shootings in US history－massacres that prompted nationwide soul-searching but did not result in major changes to the country's lax gun ownership laws.

In 1999, two boys shot and killed 12 classmates and a teacher at Columbine High School.

Then in 2012, a heavily armed man stormed a movie theater in Aurora, murdering 12. The gunman is now serving a life sentence without the possibility of parole.

United Nations Secretary-General Antonio Guterres on Monday expressed his profound concern over the rise in violence against Asians and people of Asian descent in the US during the COVID-19 pandemic.

The world has witnessed horrific deadly attacks, verbal and physical harassment, bullying in schools, workplace discrimination, incitement to hatred in the media and on social media platforms, and incendiary language by those in positions of power, Farhan Haq, deputy spokesman for Guterres, said in a statement.

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The secretary-general expressed his full support for the victims and families, and stood in solidarity with all those facing racism and other assaults on their human rights.

"This moment of challenge for all must be a time to uphold dignity for all," the UN chief said.

Liu Yinmeng in Los Angeles, agencies and Xinhua contributed to this story.