More than 30 supporters were arrested in violent clashes that followed a tense cup match between English Premier League rivals Everton and Manchester United, police said on Sunday.
One officer was hospitalised and four others injured as groups of rival fans pelted each other with stones and bottles in Everton Valley following United's 2-0 win in an FA Cup fifth round tie in Liverpool on Saturday.
"There were running clashes between rival supporters and the aggression was turned towards the police," a Merseyside police spokesman said.
"Police made 33 arrests, mostly for public order offences and drunk and disorderly behaviour," he added.
Violence also flared during the match when Manchester United goalkeeper Roy Carroll was struck on the head with an object thrown from the crowd.
Police said they would study CCTV and television footage of the incident in an effort to identify the person responsible.
The match marked the return of Manchester's United's England international Wayne Rooney to his former home ground, creating heightened tension among Everton supporters even before play began.
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