Liverpool FC have condemned damaged caused to the Real Madrid team coach as hundreds of fans gathered outside Anfield.
A club spokesman said the behaviour "of a few individuals" was "totally unacceptable and shameful", and apologised for any distress caused to the players.
Merseyside Police say around 400 fans lined Anfield Road, despite advice not to attend the ground.
As the the coaches approached for the Champions League quarter-final second leg "a number of items were thrown at the Real Madrid team bus."
Flares and pyrotechnics were also set off.
Liverpool staff were seen cleaning up a pile of broken glass on the ground, although police say the outer glass of the coach was not "broken through" by fans.
No injuries have been reported.
The club has condemned the "shameful behaviour", and has promised full co-operation into a police investigation.
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.
They added: "We will work together with Merseyside Police to establish the facts and identify those responsible."
Chief Superintendent Zoe Thornton, from Merseyside Police, confirmed an investigation is underway, and condemned the "extremely disappointing" and "disgraceful behaviour".
The disgraceful behaviour of those involved this evening is extremely disappointing and totally unacceptable.
She said: "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.
"I would also ask anyone who was in the area and captured mobile phone video footage of the incident to please get in touch."
The incident happened as hundreds gathered on Anfield Road to watch Liverpool and Real Madrid team coaches arrive for the Champions League match shortly before 7pm on 14 April.
Liverpool fans lined the street as their team bus arrived, with red flares lit as it passed.
It is not the first time the opposition coach has been damaged in the build-up to a match at Anfield.
in 2018 Manchester City's bus was hit by bottles and flares, ahead of another Champions League last-eight clash.
It caused so much damage City had to call for a back-up bus to take the players home after their 3-0 defeat that night.
On that occasion UEFA fined the club €20,000 (£17,000) for the damage as well as €6,000 (£5,200) for setting off fireworks and throwing objects.