A group of Atletico Madrid fans were filmed chanting a racist song about Real Madrid forward Vinicius Junior outside the Wanda Metropolitano stadium, before the teams met in Sunday's La Liga derby clash.
"You are a monkey, Vinicius, you are a monkey," chanted a group of fans recorded by radio station Cope, which they said was 'hundreds' strong on Twitter.
Pedro Bravo, the head of the Spanish football agents association, sparked a furore on Thursday when he said on television that Vinicius needed to stop 'acting the monkey' when celebrating goals.
The comments excited outrage, particularly from big names in Brazilian football who said the remark was racist.
Bravo apologised, saying he was using a phrase which was not intended to have racist connotations. The phrase is commonly used in Spain to mean 'playing the fool'
Vinicius and Real Madrid both published statements condemning racism directed at the forward earlier in the week.
"What we have to do is not let Vinicius score a goal against us today," said Atletico Madrid president Enrique Cerezo on Sunday.
"If he does, then may he celebrate it in the correct way and with respect to all the fans."