Bukayo Saka was involved in a furious altercation with Real Madrid's Dani Carvajal at half-time as local police intervened.
Real and Arsenal went into the break goalless following a thrilling 45 minutes at the Bernabeu, which saw Saka fluff his lines from the spot after attempting a Panenka and VAR deny the hosts a spot-kick.
Match referee Francois Letexier overturned his original decision following a lengthy review as UEFA released a statement.
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As both sets of teams headed down the tunnel, Saka clashed with Carvajal, who isn't even playing.
A video shows Carvajal speaking to the England winger before putting his hands on him.
Saka clearly didn't appreciate Carvajal's gesture as Arsenal officials intervened to defuse the situation.
Local police were also at the scene of the incident, with Ben White, who is on the bench, confronting the injured Real defender.
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It appears Carvajal was trying to get in Saka's head ahead of the second-half, having had his penalty saved by Thibaut Courtois in the opening stages of their second-leg tie.
The Gunners star will be hoping his missed-penalty does not prove crucial as Martin Keown bemoaned the players's decision.
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On TNT Sports, Keown said: "Bukayo Saka just gets it completely wrong. That was a massive opportunity to put this to bed."
Former Arsenal defender Matthew Upson echoed Keown's sentiments on Saka's penalty.
On BBC Radio 5 Live, he added: "Business-like is a good way to describe that first half performance from Arsenal.
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"Team organisation, understanding of what's required, managing the game, they've done all those things really well.
"They've had a bit to deal with but also created a bit themselves.
"They should be 1-0 up. The penalty selection from Bukayo Saka I can't quite understand but they're in a great position and they'll be really happy to be going in at 0-0 in this game."
Topics: Bukayo Saka, Arsenal, Real Madrid, Champions League, Football
Nasir Jabbar