A video surfaced on social media showing what were suspected Nazi salutes and monkey gestures from the away section of Emirates Stadium.
"We utterly condemn such behavior," an Arsenal spokesman said after the club's first-leg match Thursday, as reported by BBC Sport. "It has no place in society or football. We are working with Valencia on this and continue to encourage fans to report incidents and to provide witness statements so effective action can be taken."
BBC Sport understands six Valencia fans were arrested during the match relating to foul and abusive language and alcohol-related issues. The incident which emerged online reportedly resulted in no arrests.
"These blatant racist gestures are shocking and inexcusable," Kick It Out stated while quote-tweeting the video.
"We hope the relevant authorities identify the perpetrators and take the strongest possible punishment.
"There is no place for any form of discrimination in football or society."
UEFA is yet to reveal its new disciplinary cases following the midweek fixtures in the Champions League and Europa League.