The police is responding to reports of a live shooter near a supermarket store in south Boulder. Officers were called to the King Soopers at 3600 Table Mesa Drive at around 2.30pm local time (8.30pm GMT).
A police source told ABC news that officers responded to reports of a person opening fire in a car park.
One eye witness recalled hearing three loud bangs as terrified shoppers fled to safety.
He told CBS: “Heard a loud bang… thought someone dropped something.
“Then by the third bang everyone was running.”
Aerial footage shows dozens of officers, police vehicles and around six ambulances in the parking zone.
At least two air ambulance helicopters have also been deployed to the area.
Boulder Police Department has urged people to stay away and said Public Information Officers (PIO) are being sent to the scene.
In a post on Twitter, the force wrote: “ALERT: Active Shooter at the King Soopers on Table Mesa.
“AVOID THE AREA. PIO is en-route.”
Joe Neguse, US Representative for Colorado’s 2nd congressional district, wrote on Twitter: “Praying for the entire #Boulder community & all of the first responders and law enforcement responding to this terrible incident.”
In an update shortly before 4pm (10pm GMT) Boulder Police said the situation remains “active”.
In a post on Twitter, the force said: “This is still an active scene. Please avoid the area.”
This is a developing news story, more to follow.