Jeremy Corbyn has appealed for calm after a brick was thrown through the window of the constituency office of Labour leadership challenger Angela Eagle.
The Labour leader said it was "extremely concerning" that Ms Eagle had been the victim of a threatening act and there should be no place for violence in politics.
In a statement, Mr Corbyn said he had also received death threats and urged party members to treat each other with "respect and dignity".
Merseyside Police are investigating a report of criminal damage in Manor Road, Wallasey, where Ms Eagle's office is located.
Merseyside Police and Crime Commissioner Jane Kennedy called it a "cowardly" offence but said it would only make Ms Eagle more determined.
Speaking outside the constituency office in Wallasey, Mrs Kennedy said:
The former Labour MP for Liverpool Broadgreen condemned the actions of those behind the damage and said the ongoing intimidation towards Ms Eagle - some of which is homophobic - would not be tolerated.