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ISIS supporters celebrated online overnight following the blast in Manchester Arena which claimed the lives of at least 19 people and wounded more than 50 others.
Nobody has yet claimed responsibility for the blast which struck as 21,000 fans were leaving the venue at the end of the Ariana Grande concert.
Two messages, posted four hours before the attack, appeared to predict the atrocity.
Twitter suspended dozens of accounts, but other euphoric supporters posted messages across Telegram and other secured messaging systems.
Prime Minister Theresa May will chair a meeting of the emergency Cobra committee at 9am.
One message on Twitter posted by a user called Abdul Haqq said: 'It seems that bombs of the British airforce over children of Mosul and Raqqa has just came back to #Manchester.'
Others shared messages encouraging so-called 'lone wolf' attacks as well as videos threatening the west and the United States.