An arrest has been made after a Leeds fan allegedly racially abused the Arsenal bench during the first half of Saturday's 4-1 home defeat in the Premier League.
Just after the half-hour mark, the Arsenal subs called over the fourth official and the Premier League has confirmed an allegation of racist abuse was reported by the visitors, which it is now investigating.
The PA news agency understands the Football Association has also opened an investigation into the incident.
West Yorkshire Police have opened their own probe after making an arrest in connection to the complaint.
A Leeds statement read: "Leeds United can confirm that an investigation into claims of racism during the first half of our game with Arsenal today are underway and that one arrest has been made in connection to the allegation.
"Racism will not be tolerated at Leeds United and any supporter found to be using racist language will be subject to a lifetime ban for all Leeds United games."
'It is a single person' - Mikel Arteta
Arsenal boss Mikel Arteta told Sky Sports after the 4-1 win it was one member of the crowd who was allegedly being racist.
"It was reported and the stadium manager will have to deal with that with the authorities," he said.
"Very disappointed because we have done so much in football to try to avoid it, but it is a single person. I don't think that merits that a full stadium pays the price.
"It was a single incident and it has to be reported. It was reported, and now the authorities will have to work with the clubs to understand what happened."
Kick It Out praise 'ally' Rob Holding
Arsenal defender Rob Holding, who reported the alleged incident to the officials at Elland Road, has been praised by Kick It Out for his allyship.
Football's equality and inclusion organisation said in a statement on Sunday: "We are concerned by reports of racism directed at Arsenal players at Elland Road last night.
"We send our solidarity and support to those impacted by it, but we also commend Rob Holding for so swiftly reporting it to officials.
"His actions are a great example of taking a stand and being an ally to his team-mates.
"We await the outcome of the investigation by Leeds and the relevant authorities, and will support, however we can, efforts to hold those potentially responsible to account."
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Kick It Out is football's equality and inclusion organisation - working throughout the football, educational and community sectors to challenge discrimination, encourage inclusive practices and campaign for positive change.