Marcelo was warming down after the Copa del Rey semi-final first leg at the Santiago Bernabeu when Sky Sports' Spanish pundit Graham Hunter reports hearing monkey chants aimed at the 25-year-old Brazilian.
Hunter was scathing in his criticism of the fans allegedly involved in the abuse, which he also claims included vile chants aimed at his son.
"The reserve players from Madrid were out, running up and down, doing the warm-down and Marcelo was one of them," Hunter said.
"Atletico fans were kept in the stadium, quite a common occurrence in England and Spain, and it was at that point they started to make monkey noises towards Marcelo.
"They echoed around the empty stadium and I think for all of us who love Spanish football, it's time for us to speak out against this.
"Marcelo turned hands on hips, looked up at them, way up at the top of the stands here, shook his head in bewilderment and didn't react.
"The monkey noises continued, louder and louder. It so happened the warm-down was finishing and a young child, who I take to be Marcelo's child, was on the sidelines and ran into his arms. The Atletico fans then sang 'Marcelo is a monkey' and also sang 'we hope your dad dies'.
"The game was rumbustious, it was a derby, but by the end the players all walked off and shook hands - it was all professional - and then there was the clear cut, unacceptable, exhibition of the type of racism some fans in Spain still think is acceptable.
"I don't want to pre-judge (any punishment). There isn't a cultural awareness here that there is in Britain, I don't feel. There were only a few of us in the media gallery who were able to hear it.
"I'm not certain whether Marcelo will ask Real Madrid to act and demand action for this, but I hope by bringing it to the attention of the authorities...that they will begin to clamp down on fans, punish them and educate them."