She then launched into an upbeat performance of Side To Side.
The US star said after meeting the 15-year-old's mum, Charlotte Campbell, from Bury, this week, she had to rearrange the One Love Manchester concert.
An emotional highlight came when Grande joined the Parrs Wood High School Choir for a version of her song, "My Everything", which had audience members, performers and many of those watching the live stream in tears.
Ariana also performed the song "Better Days" with her tourmate Victoria Monet.
"And just I was about to leave the room, Adam looked at me and he said, "hey Scooter, make sure you tell them don't go forward in anger, love spreads".
"There's other concerts I want to go to in the future without being as anxious", she added.
"We will not be silenced", she yelled before singing the anthem, "Roar". "Would you agree that love always wins?" "We plan to honor them with courage, bravery and defiance in the face of fear".More news: Cricket fans observe silence for London attack victims
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Sunday's concert will raise money for victims of the suicide bombing that struck at Grande's show. Last month, an explosion killed 22 fans and injured over 100 at the end of Grande's concert in Manchester, England.
Grande performed throughout the show, even collaborating with others onstage.
During his performance, Bieber also started a chant of the word "love" among the crowd. "Incredible", said one incredulous viewer.
"I don't feel or smell or hear or see any fear in this building".
"All we feel is love, resilience, positivity".
A sell-out crowd of 50,000, many of them clutching "For our angels" signs, fell silent for a minute before Mumford & Sons frontman Marcus Mumford opened the show.
Robbie Williams performs Angels at One Love Manchester.
The One Love Manchester benefit concert organized by the "Into You" singer was a lot of things: a star-studded, feel-good show; an emotional and at-times heart-wrenching tribute to the victims and survivors of recent United Kingdom terror attacks; and a show of solidarity, strength and love in the face of hate and divisiveness.
Some fans questioned whether Sunday's show would go on after three men drove a van into a crowd on a London bridge Saturday night before leaping out and stabbing numerous people in nearby bars and restaurants.