Rodman was in Pyongyang on a trip with the Harlem Globetrotters.
Kim Jong-un, who is a basketball fan, was told by Rodman that he had made "a friend for life."
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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.