Liverpool have condemned the "unacceptable and shameful behaviour" of "a few individuals" after Real Madrid's team bus had a window smashed as it arrived at Anfield on Wednesday.
Merseyside Police said 400 people were on Anfield Road before the Champions League quarter-final second leg.
The Real bus was booed as it passed, with several objects thrown.
One pane of a double-glazed window was smashed, with Liverpool staff seen cleaning up broken glass on the ground.
A Liverpool spokesperson said: "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.
"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."
Merseyside Police said: "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.
"As with all matches, we have worked very closely with both clubs to ensure the safety of the public and the teams themselves, and the disgraceful behaviour of those involved this evening is extremely disappointing and totally unacceptable.
"Such incidents will not be tolerated under any circumstances. A dedicated investigation team will be reviewing CCTV footage from the area, along with our own body worn camera footage, and working closely with the club to identify those responsible."
La Liga giants Real lead 3-1 on aggregate, with the winners of the tie to face Chelsea in the last four.
The match is being played behind closed doors because of ongoing Covid-19 restrictions.
In 2018, cans, bottles and flares were thrown at Manchester City's team bus as a crowd gathered outside Anfield before Liverpool's 3-0 Champions League quarter-final first-leg win.
At the time, Liverpool apologised and called the behaviour "completely unacceptable". No players or staff were injured but two police officers were hurt and the coach was left unsafe to be driven.
They were fined 20,000 euros (£17,000) for the damage, plus 6,000 euros (£5,200) for setting off fireworks and throwing objects.
- Snowfall box set: Hard-hitting drama on the epidemic that hit 1980s Los Angeles
- The 'voice of golf' Peter Alliss: Revealing the charismatic man behind the microphone