One officer was standing guard at the entrance to the police station in the north of the city this afternoon.
An ambulance was called there at 11am after an officer was killed in an incident with a firearm.
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Witnesses reported hearing a "huge bang" at the venue during the concert on Monday evening.
ITV News travels to Britain's eastern-most city to gauge how the electorate will be casting its votes.
Carefully choreographed campaigns can become unstuck because voters have the power to peel back a politician's veneer.