Real Madrid have begun an investigation after a supporter inside the Bernabeu was filmed appearing to perform a Nazi salute in the moments before Wednesday's game against Benfica.
The club said the fan was ejected from the ground after being identified by stadium security.
A statement said the club have asked their disciplinary committee "to initiate an immediate expulsion procedure" for the offender.
It continued: "Real Madrid condemns this type of gesture and expression that incites violence and hatred in sports and society."
The incident came a week after Brazilian forward Vinicius Junior made an allegation he had been racially abused by Benfica's Gianluca Prestianni during the first leg of the Champions League play-off round tie in Lisbon, which UEFA is investigating.
Prestianni, who denies the allegation, was handed a provisional one-match ban and did not take part in Wednesday's game.
Real won 2-1 on the night and 3-1 on aggregate to reach the last 16 with Vinicius scoring the second-half winner.
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