Violent scenes reportedly marred the build-up to the Champions League clash between Real Madrid and Legia Warsaw on Tuesday night.
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.
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.
