Buffalo Bills safety Damar Hamlin suffered a cardiac arrest during the NFL game against the Cincinnati Bengals and is in a critical condition in hospital.
Hamlin was given CPR on the field after colliding with Bengals wide receiver Tee Higgins before being taken to hospital in Cincinnati.
"Damar Hamlin suffered a cardiac arrest following a hit in our game versus the Bengals," the Bills posted on their official Twitter account.
"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."
Bills players and staff were seen praying together on the field as an ambulance took 24-year-old Hamlin to hospital.
Hamlin's team-mates were given a standing ovation by fans and NFL commissioner Roger Goodell soon announced that the game had been postponed following the incident.
In a statement, the NFL said: "Hamlin received immediate medical attention on the field by team and independent medical staff and local paramedics.
"He was then transported to a local hospital where he is in a critical condition."
A statement from the NFL Players Association said: "The NFLPA and everyone in our community is praying for Damar Hamlin.
"We have been in touch with Bills and Bengals players, and with the NFL. The only thing that matters at this moment is Damar's health and well-being."
There was an outpouring of emotion across the United States in response to an incident which came in the penultimate week of the NFL's regular season.
Higgins tweeted: "My prayers and thoughts go out to @HamlinIsland the Hamlin Family. I'm praying that you pull through bro. Love." Higgins added a prayer and blue heart emoji.
Bills quarter-back Josh Allen said on his personal Twitter account: "Please pray for our brother."
Arizona Cardinals' JJ Watt, a three-time NFL defensive player of the year tweeted: "The game is not important. Damar Hamlin's life is important. Please be ok. Please."
Buffalo are the only franchise to play in New York state and Eric Adams, the Mayor of New York City, led the prayers for Hamlin.
Adams said: "I'm asking all New Yorkers to join me in praying for Damar Hamlin, his family, his team-mates, and our brothers and sisters in Buffalo."
Basketball star LeBron James also offered has prayers to Hamlin following the Los Angeles Lakers' victory over the Charlotte Hornets on Monday night.
James said: "My thoughts and super prayers go up to the skies above for that kid's family, for him, for that brotherhood of the NFL and everybody part of the NFL family.
"It was definitely the right call to suspend that game. The safety of players in all sports is always the most important.
"It was a terrible thing to see and I wish nothing but the best for that kid, for the city of Buffalo, the franchise of the Bills, the rest of the NFL and the Bengals who were playing in that game."