Bolton Wanderers midfielder Fabrice Muamba was left critically ill after collapsing during an FA Cup quarter-final tie against Tottenham.
The 23-year-old was rushed to the intensive care unit of the heart attack centre at the London Chest Hospital.
Medics spent six minutes trying to resuscitate him on the field after he fell to the ground with no other players around him.
"Bolton Wanderers can confirm that Fabrice Muamba has been admitted to The Heart Attack Centre at The London Chest Hospital where he is in a critically ill condition in intensive care," said a joint statement from Bolton and the hospital released at 2130 GMT on Saturday.
"No further information will be issued at this stage. The club and hospital request that the media and public respect the family's privacy at this time."
Medical staff gave the former England Under-21 international mouth-to-mouth resuscitation and tried to revive him with a defibrillator.
In total six medics were treating the player, who joined Wanderers from Birmingham in 2008 and has made 148 appearances for the club.
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.
After Muamba was taken to hospital, the match was abandoned.
The former Arsenal midfielder was accompanied in the ambulance by Coyle and club captain Kevin Davies.
Football Focus presenter Dan Walker had an exchange of text messages with Davies on Saturday evening.
"Encouragingly, he [Davies] said Fabrice is showing real fight which is the best possible news," Walker said.
"Kevin says he is not giving up and he's digging in and showing a bit of battle.
"The fact they got him breathing again makes you marvel at the work these paramedics, doctors and nurses do."
Arsenal defender Johan Djourou, who played with Muamba at the Gunners, posted on Twitter that he had visited his friend in hospital.
"Just left the hospital. Love you so much man! Keep fighting. Everybody please pray for him he's an amazing man and friend."
BBC Radio 5 live's chief football reporter Ian 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 it was a "terrible thing to see for all the football family".
"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," he said.
Football Association chairman David Bernstein said their thoughts and prayers were with Muamba and his family.
"Fabrice has played 33 times for England Under-21s, captaining Stuart Pearce's side during this time and is a player, and more importantly, a person we care greatly for," he said.
"We are in contact with Bolton Wanderers over Fabrice's condition and are awaiting updates."
Premier League chief executive Richard Scudamore added: "The thoughts of the Premier League, its clubs and players are with Fabrice Muamba, his family and Bolton Wanderers.
"We would like to praise the players, match officials, coaching staff and medical teams of both clubs at White Hart Lane for their swift actions in attending Fabrice.
"The League would also like to commend the compassion shown by the fans of Bolton Wanderers and Tottenham Hotspur.
"We hope to hear positive news about Fabrice who is and has been a wonderful ambassador for the English game."