Merseyside Police said they have launched an investigation after the bus carrying Real Madrid players was damaged as it arrived at Anfield for Wednesday's Champions League quarter-final second leg against Liverpool.
A video posted on Instagram showed Liverpool fans, some holding flares, booing the arrival of the bus. A video on Twitter showed a section of the bus's outer window frame had been damaged.
"We can confirm an investigation is underway after damage was caused to the Real Madrid team bus ...," chief superintendent Zoe Thornton said in a statement.
"Despite the advice to avoid attending the ground, approximately 400 people attended Anfield Road and as coaches approached, a number of items were thrown at the Real Madrid team bus.
"We are aware of images which show the outer pane of the lead coach window being damaged, the glass did not break through and there are no reported injuries.
"A number of pyrotechnics were also lit in the area, which significantly reduced visibility and could have put other people in danger."
Liverpool apologised to their Spanish opponents before the game for the incident.
"We condemn unequivocally the actions that led to Real Madrid's team bus being damaged during its arrival to Anfield this evening. It is totally unacceptable and shameful behaviour of a few individuals," the English club said in a statement.
"We sincerely apologise to our visitors for any distress caused."
Real progressed to the semi-finals of the European competition after they held on for a goalless second-leg draw on Wednesday to complete a 3-1 aggregate win.