Hamlin was administered CPR on the field, ESPN reported, and he was surrounded by teammates, some of them in tears, after he was hurt while tackling Bengals receiver Tee Higgins.
Higgins led with his right shoulder, which hit Hamlin in the chest. Hamlin then wrapped his arms around Higgins' shoulders and helmet to drag him down. Hamlin got to his feet, appeared to adjust his face mask with his right hand, and then fell backward about three seconds later and lay motionless.
The NFL announced the Bills-Cincinnati Bengals Monday Night Football game has been postponed.
Buffalo Bills team trainers got to Damar Hamlin within 10 seconds after he collapsed on the field mid-game, and an ambulance was on the field less than five minutes afterward, video footage shows.
Hamlin received immediate medical attention by "team and independent medical staff and local paramedics," the NFL said in a statement.
Here is what Hamlin's best friend said a few hours after the incident: