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"This is currently being treated as a terrorist incident until police know otherwise," the Greater Manchester Police said on Twitter.
"Broken. From the bottom of my heart, I am so so sorry. I don't have words," Ariana Grande tweeted.
Social media accounts say a couple loud bangs were heard at the arena where the singer was performing. Joseph Carozza, a representative from Grande's US record label, said the singer is OK and they are investigating what happened.
"A huge bomb-like bang went off that hugely panicked everyone and we were all trying to flee the arena," concertgoer Majid Khan, 22, told Britain's Press Association. "It was one bang and essentially everyone from the other side of the arena where the bang was heard from suddenly came running towards us as they were trying to exit."
Added Oliver Jones, 17: "The bang echoed around the foyer of the arena and people started to run."
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Chaos after explosion at Ariana Grande concert.
The Dangerous Woman Tour is the third concert tour by Grande to support her third studio album, Dangerous Woman. The tour began on February 3, 2017, in Phoenix, Arizona at the Talking Stick Resort Arena.
From Manchester the tour is to move through Europe, including Belgium, Poland, Germany, Switzerland and France, through the summer with stops in Brazil, Chile, Argentina, Costa Rica, Mexico and on to Japan, Thailand, the Philippines, New Zealand, Australia and more.
The Associated Press contributed to this story.
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