Bolton Wanderers midfielder Fabrice Muamba collapsed and stopped breathing during the FA Cup fifth-round tie against Tottenham.
Medics spent 10 minutes trying to resuscitate the 23-year-old on the field after he fell to the ground with no other players around him.
BBC Radio 5 live's Ian Dennis says the former England Under-21 international is fighting for his life.
The score was 1-1 when the match was abandoned after 41 minutes.
Medical staff gave Muamba mouth-to-mouth resuscitation and tried to revive him with a defibrillator. In total six medics were treating the player.
Both sets of supporters chanted Muamba's name as he was taken off the pitch on a stretcher. ESPN, who were broadcasting the match, reported that he was not breathing as he was taken into the tunnel.
Referee Howard Webb called the visibly shaken players off the field after consulting with Bolton manager Owen Coyle and Tottenham boss Harry Redknapp.
Minutes later the match was abandoned.
PLAYERS' REACTION ON TWITTER
"Praying for you Fab. Hope he's OK. Thoughts with him and his family. For all those asking, I know as much as you do. Waiting anxiously for updates from teammates. Fab is a fighter!" Bolton midfielder Stuart Holden
"Pray for Fab. God willing he will pull through."Tottenham striker Jermain Defoe
"All our hearts with Fabrice Muamba, one of my closest friends at Bolton. I'm shocked, wishing him a fast recovery." Wigan's former Bolton goalkeeper Ali Al Habsi
"Hope Fabrice Muamba is OK. Praying for him and his family. Still in shock." Manchester United and England striker Wayne Rooney
"Doesn't matter who you support. Doesn't matter if you aren't a football fan. Doesn't matter if you aren't religious. Pray for Fabrice Muamba."Tottenham defender Kyle Walker
"I'm so sad about what happened to Fabrice Muamba today. Played with him for a couple of years. What a great guy. Always a smile on his face. Please Fabrice bring that smile back. My thoughts are with you and your lovely family!"Arsenal captain Robin van Persie
A Bolton spokesman said: "Fabrice Muamba has been taken to hospital following his collapse. There is no further information at this time."
The former Arsenal midfielder was accompanied in the ambulance by Coyle and club captain Kevin Davies.
BBC Radio 5 live's chief football reporter Dennis, who was covering the game at White Hart Lane, witnessed medics rushing to aid Muamba.
"When the medical staff arrived his face was on the turf," he said.
"You could see them using a defibrillator and he was [electronically] charged on at least two occasions. Everyone could see Fabrice Muamba was fighting for his life.
"If he does pull through it will be down to the quick actions of the medics."
Soon after the incident, messages of support flooded in for the former Birmingham player.
Spurs' Rafael van der Vaart was one of those on the pitch when Muamba collapsed. He tweeted: "Terrible what happened with Muamba during the game. We're all praying for him."
England international Jack Wilshere, who played with Muamba at Bolton, wrote: "Hope Muamba is OK. Thoughts with him."
Middlesbrough defender Justin Hoyte, who played in the Arsenal youth team with Muamba, said: "I seriously hope my best friend in football is OK. Stay strong bro, please please stay strong. God is with you remember that."
Professional Footballers' Association chief executive Gordon Taylor said: "At the moment we are all watching, waiting, hoping and praying. For all the football family it's a terrible thing to see.
"This can happen on a pitch and [it is] one of the reasons why we insist all youngsters coming into the game have the ability to have heart screenings to avoid this happening."
Muamba joined Wanderers from Birmingham in 2008 and has made 148 appearances for the club.
MUAMBA'S CAREER
Born on 6 April 1988 in Kinshasa, Democratic Republic of Congo
Progressed through Arsenal's youth academy, representing England at every level from under-16 to under-21
Joined Birmingham in 2007, having been there on loan previously, then moved to Bolton the following year
Appeared at the 2011 European Under-21 Championship in Denmark for England Under-21s, for whom he made 33 appearances
