10:35 AM PT -- Sources with direct knowledge tell TMZ ... Ford was transferred to the Mayo Clinic in Scottsdale Friday. He is stable, but still in critical condition.

Country music star Colt Ford gave no inkling that he was feeling ill while he performed on stage just moments before he suffered a heart attack.

TMZ obtained video of the 17th and final song Colt cranked out during his 2.5-hour set at Dierks Bentley's Whiskey Row in Gilbert, Arizona Thursday night.

While playing "Cricket On A Line," Colt seemed vibrant and strong, belting out the lyrics while swaying to the music. At other points during the tune, Colt raised his arms in the air with the crowd and lifted the mic stand over his head.

After he wrapped up the number, Colt took a bow with his band members and exited the stage seemingly in good health.

But then everything went downhill for Colt, who suffered a heart attack, prompting police and paramedics to respond.

Check out our footage showing all the cops and fire trucks outside the venue, which was cordoned off with police tape.

Our sources tell us an officer yelled that someone had passed out behind the bar and security ran inside the joint. Presumably, the cop was referring to Colt, but he didn't specifically say who it was.

Colt was eventually transported to the ICU at the Banner Desert Medical Center in Mesa, Arizona. His rep told us that Colt is in stable condition.