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 only issue regarding the crowd that was written down in the referee's report was objects thrown onto the pitch, specifically an umbrella that was thrown from the stands while Barca celebrated Messi's goal.
Referee Mateu Lahoz picked it up before giving it to the fourth official. In his report the referee wrote: "Several objects were thrown, without impacting any player, highlighting a black umbrella."
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.
Atletico Madrid spokesman Clemente Villaverde felt the home fans had not crossed the line of what is acceptable.
"Atletico is always against any sign of violence or incorrect behaviour," he said. "The public has kept a normal behaviour within the course of a game like this one."
Barca coach Ernesto Valverde praised Griezmann in the post-game news conference: "I saw him well on the pitch. It was a difficult game in terms of the emotional aspect. The crowd did boo him a little but I saw him well. He worked and fought to help the team."
During five years with Atletico, Griezmann won the Europa League, the UEFA Super Cup and the Spanish Super Cup and scored 133 goals in 257 appearances.