The incident prompted an outpouring of support from the NFL community and beyond prior to an announcement from the Bills that Hamlin had suffered cardiac arrest.
"Damar Hamlin suffered a cardiac arrest following a hit in our game versus the Bengals," the Bills announced in a statement early Tuesday. "His heartbeat was restored on the field and he was transferred to the UC Medical Center for further testing and treatment. He is currently sedated and listed in critical condition."
Hamlin, 24, collapsed after a collision with Cincinnati Bengals wide receiver Tee Higgins on a pass play. Hamlin stood up after completing the tackle, but almost immediately fell back to the turf.
He remained on his back as players surrounded him and called for medical attention. An ambulance drove on to the field and medical staff spent several moments tending to him as he laid on the turf. He required CPR on the field, according to reporters on the scene, and was eventually transported to a local hospital. More than 15 minutes passed before he was driven off the field in the ambulance.
The NFL eventually postponed the game and confirmed that Hamlin was in critical condition. His marketing representative Jordan Rooney tweeted later Monday that Hamlin's vitals were back to normal and that he was administered a breathing tube. The Bills then announced early Tuesday the diagnosis of cardiac arrest.
As the scene played out, members of the NFL community reacted on TV and on social medial, including former NFL safety and current ESPN analyst Ryan Clark.
"This is about Damar Hamlin," Clark said. "It's about a young man at 24 years old who was living his dream that a few hours ago was getting ready to play the biggest game of his NFL career. There's probably nowhere else in the world he wanted to be.
"Now he fights for his life. And when Damar Hamlin falls to the turf and when you see the medical staff rush to the field and both teams are on the field, you realize this isn't normal. You realize this isn't just football. ... We forget that part of living this dream is putting your life at risk. And tonight, we got to see a side of football that is extremely ugly."
Former Pittsburgh Steelers linebacker Ryan Shazier was paralyzed on a tackle on the same Cincinnati field in 2017. He tweeted his support for Hamlin.
Higgins likewise expressed his well wishes.
As did members of the larger football community, including current and former NFL players.
"My thoughts and super prayers go up to the skies above for that kid's family, for him," James said. "... It's a terrible thing to see, and I wish nothing the best for that kid, the city of Buffalo, the franchise, the Bills and the rest of the NFL too as well and also the Bengals that were there playing that game."
"Prayers up for the Bills safety," Mitchell said. "... I wish him and his family — we're praying for him, all of us in the locker room. Kevin Love actually hit me on it and wanna — from the Cavaliers organization — we wanna wish the best and pray everything goes well."
Fans of both teams gathered at the hospital where Hamlin was receiving treatment for a candlelight vigil.
NFL reporter Lisa Salters was on the scene covering the incident. She struggled to hold back tears during her report.
"We've grown accustomed to it, seeing guys take hard hits," Salters said. "See them stay down for a little while, get back up, give the thumbs up. That's what we were all hoping for was that Damar Hamlin was gonna get up and get in that ambulance, he was gonna give us the thumbs up. We were all gonna know he was OK.
"When that didn't happen, I think this entire stadium was devastated. All I can really think about is that player. His teammates just seeing the agony on their faces, the concern on their faces. They're scared for him right now and they should be. We all should be."