Greater Manchester Police arrested one person after alleged tragedy chanting during the FA Cup quarter-final between Manchester United and Liverpool.
Offensive songs were allegedly heard during United's 4-3 extra-time win over their rivals at Old Trafford on Sunday.
"We are all familiar with fans taunting their opponents ... but this is generally done in good humour and without offence," said police.
"The incidents today crossed the line of what is acceptable."
"Incidents of tragedy chanting will not be tolerated and will be dealt with robustly," read a statement, adding: "We will work closely with both clubs to identify anyone responsible for committing offences during the event.