Xavi's side were 2-0 ahead when players and officials were alerted to the incident in the stand behind one of the goals.
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The game was subsequently halted in the 81st minute with players from both teams watching on with concern for the spectator.
Barcelona defender Ronald Araujo and Cadiz star Fali were seen praying during the delay as it became clear the situation was extremely serious.
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Cadiz goalkeeper Conan Ledesma was then seen sprinting across the pitch carrying a defibrillator that he then threw into the crowd to help resuscitate the fan.
Ledesma attempted to communicate with fans in the stand by raising his thumb in an attempt to obtain some good news but the lack of response told its own story with the players eventually being led back to the changing rooms.
Cadiz midfielder Jose Mari was later seen running down the touchline with a stretcher before accompanying a Red Cross worker to the supporter who had taken ill.
The game was suspended for an hour in total before getting back underway after Cadiz President Manuel Vizcaino told a Spanish radio station that the fan had been successfully resuscitated and taken to hospital.
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A shell-shocked stadium watched on as the two teams finished the final nine minutes of the game with Barcelona adding another two goals in the dying minutes.
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Goals from Ansu Fati and Ousmane Dembele added to earlier strikes from Frenkie de Jong and Robert Lewandowski to secure an eventual 4-0 win.
The win sees the Catalan giants maintain their unbeaten start to the season having won four league games on the bounce, following an opening day draw with Real Vallecano.
Los Cules kicked off their Champions League campaign in style with a 5-1 win over Victoria Plzen on Wednesday and face a first true test of their credentials at Bayern Munich on Tuesday.