Roughly six minutes remained in the first quarter when Hamlin, 24, made an open-field tackle on Bengals wideout Tee Higgins, who'd caught a pass while headed across the middle of the field.
Higgins's helmet appeared to hit Hamlin square in the upper chest area.
Hamlin got up -- and seconds later fell onto his back without moving again.
Replays were broadcast to millions watching what had begun as the most anticipated matchup of the NFL season.
Hamlin was in critical condition at the University of Cincinnati Medical Center as of 10 p.m., the NFL said in a statement. No other updates would be provided the rest of the night, league officials said.
The nature of the tackle raised fears that Hamlin may have suffered commotio cordis, which is caused by a sudden blunt impact to the chest that causes cardiac arrest.
The reaction of medical personnel, players and coaches made clear that the situation was dire -- and not the typical gathering of players, coaches and others waiting for word that their comrade is OK.
Hamlin received CPR and oxygen on the field for several minutes as players from both teams formed a human wall around the scene.
Bills wide receiver Stefon Diggs rocked back and forth, staring straight ahead, tears flowing. Other players hugged and wept openly. Bills quarterback Josh Allen and Bengals QB Joe Burrow embraced.
Hamlin was eventually placed on a stretcher and rushed from the field to UCMC in an ambulance.
As everyone participating or watching waited, coaches Sean McDermott and Zac Taylor and an NFL executive, Dawn Aponte, spoke on a cell phone in the tunnel with league officials. The announcement came soon after that the game was being postponed.
Diggs quickly took an Uber to see his teammate and friend, said an ESPN reporter at the hospital, the region's only Level 1 trauma center. Other visitors included Taylor, the Bengals coach, he said.
Outside the hospital, a small group of Bengals fans gathered in a prayer circle with their Buffalo counterparts.
ESPN commentator Ryan Clark, a retired player who once had emergency surgery to remove his spleen and gall bladder -- and nearly lost his life -- made a powerful speech.
“The first thing - this is about Damar Hamlin, it‘s about a young man at 24 years old who was living his dream,” Clark said on "The Scott Van Pelt Show."
“A few hours ago he was getting ready to play the biggest game of his NFL career and there was probably nowhere else in the world that he wanted to be," Clark said. "Now, he fights for his life.
“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.
“This isn’t about a football player. This is about a human. This is about a brother. This is about a son. This is about a friend. This is about someone who is loved by so many who have to watch you go through this.
"I think the next time we get upset at our favorite fantasy player or we’re upset that the guy on our team doesn’t make the play and we’re saying he’s worthless and you get to make all this money, we should remember these men are putting their lives on the line to live their dream," Clark said, "and tonight Damar Hamlin’s dream became a nightmare for not only himself but his family and entire team.”
Hamlin worked his way up to a starting position this year in his third season with Buffalo. He played college ball at the University of Pittsburgh before becoming a Bills sixth-round pick.
This is a developing story. Check Daily Voice for updates.
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