"Cancel Justin's concert in the UK, please! We want him to be safe, please," a person using the handle marhrukhh wrote on the Instagram account of Bieber's manager, Scooter Braun.
Braun also manages Grande, also 23, a former Nickelodeon star whose huge female fan base, many of them tweens, were out in force for her concert in Manchester on Monday night when a suicide bombing killed 22 people and injured dozens.
The "Problem" singer flew to Florida on Tuesday to spend time with her family, the Daily Mail website reported. Representatives at Grande's record company did not return calls on whether the remainder of her world tour, with stops in London and Europe, would go ahead.
Leanne Murray, 20, who lives in Ireland, has tickets to see Bieber play in Dublin next month. But she said that after Monday's bombing she is contemplating selling them.
"I just don't want what I would hope to be a great night to end in something like last night," Murray, who paid 180 Euros ($200) each for two tickets told Reuters.
"It's frightening to think that it could have been any of us and it really shows that you never know what's around the corner," Murray added.
Representatives for Bieber did not immediately respond to Reuters on the status of his upcoming performances.