"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, People magazine reported. Braun and her 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 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.
Touring has been one of the top sources of income for musicians in recent years. The top 10 worldwide tour acts grossed a combined $1.67 billion in 2016, according to music industry publication Pollstar, with Bruce Springsteen bringing in $268.3 million alone.
Pop acts like Taylor Swift, Bieber, One Direction and Grande are also among the top earners. Grande grossed more than $25 million from touring between April 2016-April 2017, according to Pollstar figures, while Bieber grossed $163 million in 2016.
"I can see with parents of young children who might be on the fence anyway about letting their 12-year-old go to concerts, that this going to put them over the edge, and they may say 'you can wait until you're older'," said Ed McPherson, an entertainment litigator and crisis manager in Los Angeles.
"It could impact some of the artists who play to younger people," McPherson added.
Joe Reinartz, news editor at Pollstar, said that although he believes security is already strong at music venues and will no doubt be stepped up, parents would likely be more cautious.
"In the short term, there are going to be concerns for any large gathering where there will be young people, and that doesn't necessarily mean a Justin Bieber show. It means a high school football game, or an event at a fair ground," Reinartz said.
Jim Donio, president of the Music Business Association, said he would be surprised to see musicians cancelling tours because of the attack at the Grande concert.
But he said fans will be more aware and prepared. "Will parents make more informed decisions? Yes, that's possible, so there can be some impact there."
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