Medics spent 10 minutes trying to resuscitate the player on the field before the players were called off by referee Howard Webb.
BBC Radio 5 live senior football reporter Ian Dennis says the midfielder is fighting for his life.
The score was 1-1 when Muamba, 23, collapsed on 41 minutes.
Medical staff gave Muamba mouth-to-mouth resuscitation and tried to revive him with a defibrillator.
He was taken on the pitch on a stretcher and ESPN reported that he was not breathing as he was taken down the tunnel.
The former Arsenal midfielder has been taken to hospital.