Jeremy Corbyn has revealed he has faced death threats this week as he spoke out to condemn the "threatening act" against Labour leadership challenger Angela Eagle.
Police are investigating reports that a brick was thrown through a window at Ms Eagle's Wallasey constituency office last night.
Ms Eagle's office also said she has also had to cancel a meeting in Luton today after the hosting hotel received threats overnight.
Helen Osgood, a senior caseworker at the office, said: "The hotel manager has had threats all night and so the meeting has not been able to go ahead."
It comes ahead of what is expected to be a bitter meeting of Labour's National Executive Council to decide the rules of the party's leadership contest.
Ms Eagle faces a vote of no confidence by members of her local party, which backs Mr Corbyn's leadership.
Mr Corbyn issued a statement in which he called for all party members and supporters to "act with calm and treat each other with respect".
Merseyside Police and Crime Commissioner Jane Kennedy, the former Labour MP for Liverpool Broadgreen, said the "cowardly" broken window would only strengthen Ms Eagle's resolve.