US president Barack Obama has suggested it is "hard to imagine" that it would be in Britain's interests to leave the European Union.
Mr Obama said the US valued the presence of its close ally at the top table in the EU and he did not believe it would be good for the 28-member bloc for Britain to leave.
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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.