Manchester United's team bus was hit by a glass bottle as it arrived at Anfield ahead of their 0-0 draw against Liverpool in the Premier League on Sunday.
Video on social media shows a bottle being thrown from the street and hitting the front of the bus as it made its way towards the stadium.
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It's reported the bottle damaged a window on one of two buses carrying United players and staff.
"Liverpool Football Club utterly condemns the actions that led to damage being caused to the Manchester United team bus during its arrival at Anfield this afternoon," the club said in a statement.
"We are aware of footage of the incident circulating online and are working with Merseyside Police, to fully investigate and identify those responsible.
"Any individuals found guilty of this reprehensible behaviour will also face the full force of the Club's sanction process."