A window on Real Madrid’s team bus was smashed after an object was thrown from a crowd of Liverpool fans gathered outside Anfield.
Hundreds of Reds fans packed onto Anfield Road before Wednesday night’s Champions League quarter-final second leg.
The Real bus was booed as it passed, with several objects thrown.
La Liga giants Real lead 3-1 on aggregate, with the winners of the tie set to face Chelsea in the last four.
One pane of a double-glazed window was smashed, with Liverpool staff seen cleaning up broken glass on the ground.
Merseyside Police officers were in attendance.