Young British men and women fighting in Syria is a "huge problem" with an estimated 500 UK citizens joining radicals abroad, Jonathan Russell told Good Morning Britain.
Political liaison officer at the anti-extremism organisation Quilliam Foundation described British men and women going to Somalia or Syria to fight as "the new face of terrorism".
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The hashtag #RoomForManchester was being widely shared.
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.