Barcelona forward Antoine Griezmann was targeted by Atletico Madrid fans with chants and banners on his first return to the club.
Barca won 1-0 thanks to a late Lionel Messi goal which was enough to see them return to the top of La Liga.
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"You wanted to have a name and you forgot to be a man," a banner in the Wanda Metropolitano's Fondo Sur read.
As Griezmann warmed up for the game on the pitch, Atletico fans chanted "Griezmann die."
And whistles greeted his name being read out over the PA system ahead of the game, while his first touch was met in a similar fashion. Griezmann's plaque outside the stadium, in honour of him playing more than 100 games for the club, had toy rats left on it prematch.
ESPN FC has contacted La Liga to ask if any action is set to be taken.
The France international's €120 million move to Barcelona in the summer was controversial.
Griezmann had publicly turned down a move to Barca in the summer of 2018 but only a year later left Atletico when the Catalan club paid his release clause.
Barca waited for Griezmann's buy-out clause to drop from €200m to €120m on July to make their move.
Atletico reported Barcelona to the Spanish Competition Committee for breaking its rules by beginning negotiations with Griezmann while he was under contract with them without their permission.
The Spanish Competition Committee fined Barca only €300 a breach of rules but absolved the player of any responsibility.
ESPN FC's Adriana Garcia contributed to this report