Liverpool apologised to Champions League opponents Real Madrid after the bus carrying the Spanish side was damaged as it arrived at Anfield for Wednesday's quarter-final second leg.
"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. We will work together with Merseyside Police to establish the facts and identify those responsible."
A video posted on Instagram showed around a hundred 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.
Real hold a 3-1 lead in the tie after winning last week's first leg in the Spanish capital. Like most elite European matches this season, the game is being held without supporters due to coronavirus restrictions.