The 52-year-old, who slammed Grande for flying home straight after the terror attack at her Manchester Arena gig on May 22, applauded the 23-year-old for putting on a "magnificent" show at the Emirates Old Trafford cricket ground in Manchester.
"I encourage you to choose love even when it's hard".
"Seeing you guys coming together was a sight to behold, we love you, we are with you, the whole world is watching", he told the crowd.
Charlotte Campbell and Paul Hodgson said they made the visit as a show of solidarity after Saturday's terrorist attack which killed seven innocent people.
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New Order rocker Peter Hook, whose daughter was hospitalised in the attack and attended One Love Manchester, told the BBC, "As a musician the last thing you ever want to do is have something like that happen at a concert where you feel ultimately responsible and I think she's done a fantastic job". "All we feel here tonight is love, resilience, positivity", Williams said.
"It's very important. We need to stand united against it now".
The large crowd had fallen silent at the start of the concert to remember those who were killed and injured in both Manchester and London.
The event is expected to bring in upwards of $3 million, which will benefit the We Love Manchester Emergency Fund, set up by Manchester City Council in partnership with the British Red Cross.
"We need to fight, we need to show that nobody is going to beat us". Backed by two singers and a guitarist, she delivered the song wearing all white, singing, "Throw your sticks and your stones, throw your bombs and your blows, but you're not gonna break my soul".
"One Love Manchester" airs Sunday, June 4, on ABC and Freeform in the U.S. It'll be live on Freeform at 2 p.m. ET and a one-hour highlight special will air on ABC after the National Basketball Association finals.
"We will not quit or operate in fear".