Video footage emerged on social media appearing to show fans punching and kicking Spanish police close to Real's Bernabeu stadium.
According to Football Espana, the violent fans are alleged to be ultras from visiting side Legia.
Paliza de seguidores del Legia de Varsovia a la Policía al lado del Bernabéu pic.twitter.com/fDhjCqsoEs— Pablo Arjona (@pabloarjona) October 18, 2016
Legia fans were at the centre of negative headlines last month when a handful of supporters allegedly threw bottles at their Borussia Dortmund counterparts during the German side's 6-0 win.
UEFA opened disciplinary proceedings against the Polish side as a result.