Manchester Together in Albert Square featured songs by Elbow and Oasis. The royal joined the families of the victims, survivors, first responders, and other emergency workers.
Gina and Casey Hankey, from Stoke, said they were at the arena.
Dean of Manchester Rogers Govender said: "In this service we come together as people of different faiths and none, as we remember with love before God those whose lives were lost, and those whose lives have been changed forever and have to live with the awful memories of that day 12 months ago".
As the crowd responded, Walsh shouted: "This is what love sounds like". The Lord Mayor of Manchester, June Hitchen, and the Bishop of Manchester, the Right Reverend David Walker, were also part of the event.
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On the altar behind him were 22 candles - fashioned from the thousands left behind at the memorial in St Ann's Square a year ago - each representing one of the young victims.
'We are Manchester, a city united'.
Nine-year-old Molly said she was taking part because it was "a good thing to do for all the people who can't be here", while Matty, 14, said the unity in singing "is what Manchester's all about". And the Manchester Lesbian & Gay Chorus performed a medley of city anthems, including New Order's Blue Monday and Wonderwall by Oasis.
One year ago today, 22 people were killed from a bombing outside Ariana Grande's concert at Manchester Arena. She returned to Manchester two weeks later for the One Love Manchester benefit concert, which she helped organise for the victims and their families.
"We are showing Manchester and the world that we carry on", said Cath Day of the Manchester Survivors Choir.
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"It's odd because I never used to have fear over anything".
Tegan said: "It was good, but I felt sorry for those who lost people, and just how lucky I am that my dad's still here".
"I must've died 200 times in my nightmares".
Later on thousands - many of them youngsters - gathered in the city centre for a "Manchester together" concert, with performances from local youth choirs, poetry and recorded messages of support from celebrities including former Manchester United star Ryan Giggs.
It also saw poet Tony Walsh call for the crowd to join him in making a "minute's noise for the 22" and "in solidarity with everyone that was injured, mentally and physically [and] for those who were first on the scene". "I love you with all of me and am sending you all of the light and warmth I have to offer on this challenging day".
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Noel Gallagher appeared on screen to introduce Don't Look Back In Anger - the track that became so popular in the days after the attack as a song of hope and defiance.