An Arsenal fan received a stream of abuse after he had walked through Tottenham fans in Bilbao with a Gunners shirt.
Spurs supporters have taken over the Spanish city in preparation for the Europa League final.
Thousands are in Bilbao, much without tickets, and there is a party atmosphere with songs that are sung and torches are dropped off.
A brave fan of Arsenal decided to have some fun with a contingent of Tottenham fans, brutally walking through a crowd with a gunners shirt on.
The Spurs fans responded by abusing the supporter and sang: “W *****! W *****! W *****!”
Up to 70,000 fans of United and Tottenham have flooded the streets of Sunny Bilbao and the nearby San Sebastian.
But scenes became sour in some areas when supporters clashed.
A large group of football hooligans was seen with objects and punches.
Witnesses say that the fight broke out when two large groups of supporters started screaming and swinging insults.
It quickly escalated in a full fight, where both United and Spurs fans could get what they could take to throw at each other.
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One hooligan destroyed a terrace table and used it as a weapon against the other group.
Drinks and bins were also hurled, creating a chaotic scene that was a serious risk for local companies and passers -by.
Various local police patrols rushed to the scene after reports of the fight and quickly recovered.