Grande is hosting the concert to raise money for those involved in the terrorist atrocity which killed 22 people and left more than 50 injured after her concert at Manchester Arena on May 22.
Grande thanked the crowd for coming out to support the event.
She said she had met with the mother of Olivia Campbell, 15, who was among those killed in the attack.
"As soon as I met her, I started crying.''
Grande said the teenager's mother told her to stop crying, that her daughter would not want her to cry.
"The kind of love and unity that you're displaying is the medicine the world really needs right now,'' Grande said.
Grande joined American group Black Eyed Peas on stage at the concert, singing their hugely popular song: Where Is The Love?
The song was first released in 2003 - when Grande was only 10 years old.
She looked on the verge of tears as she passionately sang the poignant lyrics.
"People killin', people dying', children hurt and you hear them cryin'. Can you practice what you preach? Or would you turn the other cheek?
"Father, father, father help us, send some guidance from above. 'Cos people got me, got me questionin', where is the love?"
The star-studded lineup also included Robbie Williams, Take That, Justin Bieber, Coldplay, Marcus Mumford, Niall Horan, Katy Perry, Pharrell Williams, and Little Mix.
Bieber sang hit Love Yourself and became emotional as he addressed the crowd.
"Would you agree that love always win? What a better way to fight evil with evil, than to fight evil with good.
"God is good in the midst of the darkness."
He said he wanted to honour the people who had been lost - before correcting himself to refer to them as those "taken.''
"We honour you and we love you.''
All net profits donated to the We Love Manchester Emergency Fund.
The concert follows another terror attack in London on Sunday. It is unclear if or how recent events will impact the Manchester concert or the performances there.
-Bang! Showbiz with NZ Herald
