A window on the Real Madrid team bus was smashed ahead of the Spanish giants' Champions League clash with Liverpool at Anfield on Wednesday.
"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," Liverpool said in a statement.
"We sincerely apologise to our visitors for any distress caused. We will work together with Merseyside Police to establish the facts and identify those responsible."
The match is being held behind closed doors due to coronavirus restrictions.
Three years ago several windows were smashed on the Manchester City team bus by thrown cans and bottles ahead of a Champions League quarter-final, first leg.
Madrid hold a 3-1 lead from the first leg in the Spanish capital last week.