Gerard Pique furiously confronted Clasico referee Jesus Gil Manzano after Barcelona's loss to Real Madrid on Saturday.
Barca slipped to third in La Liga following a 2-1 defeat at Estadio Di Stefano, with first-half goals from Karim Benzema and Toni Kroos wrapping up the three points for Zinedine Zidane's Madrid.
Oscar Mingueza netted a second-half consolation for the visitors, who felt hard done by after seeing Martin Braithwaite denied a late penalty, and Pique led their protests after the final whistle.