Former Arsenal and Manchester City midfielder Patrick Vieira has tweeted:
It was brave of Kevin Prince Boateng to do what he did today, and it was the right thing. We need to stand up and stand together. Well done.
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The Islamic centre where the Manchester suicide bomber is thought to have prayed says the attack "has no place in Islam or any religion".
German chancellor Angela Merkel and Indian prime minister Narendra Modi among those to praise "great spirit" of Manchester people.
The bomber's identity has been confirmed by Greater Manchester Police.