A gunman has killed 10 people - including one police officer - after opening fire at a grocery store in Boulder, Colorado, on Monday.

The incident occurred at approximately 02:30PM local time, after the man entered the King Soopers market and opened fire.

About 20 minutes later, the Boulder police department alerted local residents on social media, tweeting: "ALERT: Active Shooter at the King Soopers on Table Mesa. AVOID THE AREA. PIO is en-route."

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The police department then asked that people not broadcast "any tactical information" on social media, after several witnesses live-streamed the incident on YouTube, BBC News reports.

Following a stand-off with police, an injured and shirtless suspect was arrested.

Speaking at a press conference following the arrest, Boulder police chief Maris Herold confirmed that a suspect was in custody and was receiving hospital treatment. "I want to reassure the community that they are safe," she said.

No possible motive has been given for the shooting.

Herold also revealed that 51-year-old officer Eric Talley had died as a result of the incident. Talley had been with the Boulder police department since 2010.

"The heroic action of this officer when he responded to the scene... phone calls of shots fired in the area and a phone call about a possible person with a patrol rifle," Herold said, adding: "Officer Talley was the first on the scene and he was fatally shot."

On Twitter, the department paid its respects to the late officer, writing: "Rest In peace Officer Eric Talley. Your service will never be forgotten."

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In a follow-up statement on Twitter, the department said: "King Soopers spokesperson offers condolences, thoughts, and prayers to employees, customers, and first responders. Company will refer all questions to law enforcement."

Additionally, Herold told press that neither the police nor coroner will release the names of the other victims until their families have been notified.

Speaking about the families waiting "in anguish to hear about loved ones", Cheif Herold said: "I am sensitive to that, and we will work around the clock to get this accomplished."

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Colorado Governor Jared Polis also responded to the incident on social media, writing: "My prayers are with our fellow Coloradans in this time of sadness and grief as we learn more about the extent of the tragedy."

And per Sky News, Boulder County district attorney Michael Dougherty told reporters: "These were people going about their day, doing their shopping. I promise the victims and the people of the state of Colorado that we will secure justice."

Featured image credit: Boulder Police Department/Twitter

10 people dead after gunman opens fire on Colorado grocery store