Officials said security was bolstered for the concert, which aims to raise money for those affected by the bombing that killed 22 people and wounded more than 100 at Grande's May 22 show.
American star Grande said she was "praying for London" after the atrocity in the capital, which killed seven and left 48 injured.
These four terror attacks have happened within the same time frame as the Manchester bombings and have been no less severe.
Elsewhere, Manchester journalist Tom Eden has shared video of those queuing outside the gates singing along to Take That's Rule The World, as the band soundchecked inside the stadium.
The One Love benefit concert will serve as a reminder to us all that we're always stronger together.
He continued: "My thanks goes to those who joined us at the events today and remained patient as we carried out an increased number of searches to ensure everyone remained safe and enjoyed themselves".
Inside Manchester's Emirates Old Trafford venue, Grande's fans who attended her concert last month, had a special reserved section at the.
Bieber and Grande's manager, Scooter Braun, noted that the singer performed her first concert in Manchester exactly two years before.
While the city would have been forgiven for keeping a low profile in the following weeks the announcement of the #OneLoveManchester concert with Ariana Grande returning to headline showed a bravery from both Ariana and the city itself. Authorities have said the attack started with a van plowing into pedestrians and then involved three men using large knives to attack people in bars and restaurants at a nearby market. The pop singer visited young fans injured in the attack at the Royal Manchester Children's Hospital on Friday.
All proceeds from the show will go towards the "We Love Manchester Emergency Fund", in partnership with the British Red Cross and Manchester City Council.
"Last night, this nation was challenged, and you had a decision to make whether to come out here tonight", Braun said to the crowd, estimated at around 50,000.
"We can't let them stop us".