FA Cup match abandoned after Muamba collapses
With 41 minutes on the clock, Muamba suddenly dropped to the floor and six paramedics ran on to the pitch moments later to treat the 23-year-old.
The paramedics tried to resuscitate the player while players and fans, many of them in tears, looked on.
"There is no further information at this time."
Bolton boss Owen Coyle shouted "he's just collapsed" before running across to join the paramedics.
A defibrillator was used on the star, who has also played for Arsenal and Birmingham.
Muamba, 23, was carried off on a stretcher after receiving six minutes of treatment.
He was wearing an oxygen mask and was flanked by a number of paramedics and Bolton medical staff.
Shortly after he left the pitch, referee Howard Webb took the players off the pitch and the game was abandoned.
There was an immediate reaction on Twitter.
"God is with you remember that."
Muamba's Bolton team-mate, Stuart Holden, wrote: "Praying for you Fab. Hope he's OK.
"Thoughts with him and his family."
"Feel sick watching this, puts football and everything else into perspective."
Both sets of fans sang Muamba's name as he lay on the pitch at the north London ground.
The announcement that the game had been called off was greeted with a round of applause from both ends.
Muamba started his career at Arsenal and spent two years at Birmingham before joining Bolton in 2008.
He was born in Zaire but has represented England from Under-16 to Under-21 level.