Bolton footballer Fabrice Muamba was "critically ill" in hospital on Sunday after collapsing suddenly during his team's FA Cup quarter-final against Tottenham.
The 23-year-old Kinshasa-born player was rushed to hospital after harrowing scenes towards the end of the first half at White Hart Lane on Saturday, when the midfielder sank to the turf with no player near him.
Several paramedics were seen attempting to revive the player before he was stretchered off the field after 41 minutes with the scores at 1-1.
Muamba was wearing an oxygen mask and flanked by a number of paramedics and Bolton medical staff as he left the pitch.
Referee Howard Webb called the players off the field before an announcement shortly afterwards that the game had been abandoned.
A joint statement from Bolton and the London Chest Hospital released at 9:30pm local time (2130 GMT) Saturday said Muamba was in intensive care.
"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," the statement said.
"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."
Earlier, players, fans and millions of television viewers had watched in shock as paramedics treated Muamba on the ground at White Hart Lane.
Several players from both teams were in tears as the seriousness of the situation became apparent and fans sang Muamba's name as he was treated.
The stricken footballer's condition prompted an avalanche of concern on social-networking site Twitter, with the words "Pray for Muamba" trending hours after he was admitted to hospital.
Muamba's Bolton team-mate Stuart Holden tweeted: "Praying for you Fab. Hope he's OK. Thoughts with him and his family."
English football stars were quick to express their concern, with Manchester United defender Rio Ferdinand tweeting: "Come on Fabrice Muamba, praying for you."
Arsenal and England midfiedler Jack Wilshere, who played with Muamba during a loan spell at Bolton, added: "Hope Muamba is okay. Thoughts with him."
Middlesbrough defender Justin Hoyte - a former youth team colleague of Muamba's at Arsenal - posted: "I seriously hope my best friend in football is OK. Stay strong bro please please stay strong."
Britain's Sunday newspapers were united in their anguish for the young player. The Sun on Sunday summed up the situation with the headline: "When football no longer matters."
"Quite rightly our great game of football was forgotten, deemed a ridiculous irrelevance as the horrifying events unfolded," Rob Beasley wrote in the tabloid.
Daniel Taylor of the Guardian described the moment of realisation that Muamba had collapsed as "absolutely terrible".
"There is something deeply chilling when a young, apparently fit, professional footballer can suddenly be face down on the turf," he wrote on the newspaper's website.
The Mail on Sunday's headline said simply: "Pray for him."
Football Association chairman David Bernstein said: "On behalf of the FA, I would like to send our thoughts and prayers to Fabrice Muamba and his family this evening.
"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.
"We are in contact with Bolton Wanderers over Fabrice's condition and are awaiting updates."
Bolton manager Owen Coyle and club captain Kevin Davies were reported to have gone to hospital to join Muamba, who moved to England from the Democratic Republic of Congo at the age of 11 after his father fled the war-torn country.