Liverpool fans threw bottles and cans and aimed fireworks at the Manchester City bus as it arrived at Anfield ahead of Wednesday night's Champions League game.
No one on the bus was hurt, but a window was smashed with smoke entering the vehicle and the coach will be unusable for the return journey to Manchester.
Merseyside police said it would be opening an investigation after two officers were injured by thrown objects.
Hundreds of Reds fans lined the streets around the ground, singing songs and waving flags before the 7:45 p.m. kickoff for the first leg of the quarterfinal. The atmosphere had been vibrant with a number of people climbing up lampposts, walls and road signs to catch a glimpse of the teams as they arrived.
City arrived first but their bus became a target for a few missiles as it slowed to navigate the corner of Arkles Lane and Anfield Road, where most of the fans were gathered.
The City bus is in there somewhere 😳🔥🔴 pic.twitter.com/jBNDeDgZIz— Anfield Leak (@AnfieldLeak) April 4, 2018
A number of red flares were also let off, making visibility difficult as supporters lined the streets outside the stadium. Liverpool arrived to a raucous welcome moments later.
Sources told ESPN FC that officials from UEFA were monitoring the arrival of the bus, so discipline from European football's governing body could follow.
Liverpool manager Jurgen Klopp apologised for the fans' actions, saying before the game: "I really don't understand that. We tried everything to avoid a situation like that. When we came through it was very positive and we didn't know what had happened before -- City went ahead of us. For Liverpool I have to say sorry."
His City counterpart Pep Guardiola said he believed last year's Borussia Dortmund bus attack, in which an explosive shattered windows, would have curbed further incidents like this.
"I want to say thank you to Jurgen for his words. We did not expect this [attack] after what happened last year at Dortmund," Guardiola said. "Fortunately no one's health has been hurt. I know that is not Liverpool: I know the history of the club is much bigger than these four, five or 10 guys."
Liverpool promptly released a statement condemning the fans' actions before the game, which kicked off on time.
"The club condemns in the strongest possible terms the scenes which preceded our Champions League quarterfinal, which resulted in damage being inflicted on the Manchester City team bus during their arrival at Anfield," it said.
"We apologise unreservedly to Pep Guardiola, his players, staff and officials caught up in the incident. The behaviour of a number of individuals was completely unacceptable and the club will cooperate fully with the authorities to identify those responsible.
"The priority now is to establish the facts and offer Manchester City whatever support is necessary."
Merseyside Police match commander superintendent Paul White said:: "The officers and stewards are there to protect the public and keep them safe.
"This behaviour by a number of people who threw bottles, cans and pyrotechnics towards the bus is completely unacceptable and we will conduct enquiries to identify who was responsible and bring them to justice.
"We worked very closely with both clubs to ensure the safety of the public and the teams themselves, and it is disappointing to see that a number of people behaved in this appalling way."
Information from ESPN's Manchester City correspondent Jonathan Smith and Press Association was used in this report.