More than 10,000 people attempted to falsely claim free tickets to the One Love Manchester benefit concert.
Ticketmaster put aside passes for the 14,200 people who attended Ariana Grande's Manchester Arena gig last week, after which 22 people were killed in a terror attack.
However, more than 25,000 people applied for free passes to the event being held to support grieving families and victims.
A Ticketmaster spokesperson said: "Sadly, over 10,000 unscrupulous applications have been made."
Joining Ariana Grande on the bill at Old Trafford cricket ground will be Justin Bieber, Katy Perry, Coldplay, Take That and Miley Cyrus.
About 50,000 people are expected to attend and tickets sold out in under 20 minutes.
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Secondary ticket sites such as Viagogo have agreed not to allow any reselling.
Meanwhile, Ticketmaster vowed to cancel any tickets being touted online after dozens were listed at inflated prices on social media and eBay.
In a statement, it said: "At Ticketmaster we are doing everything we can - including extending today's deadline - to ensure that tickets go to the actual fans and not the opportunists or touts who have also been applying for free
"We're working hard to get tickets direct to fans. We are cancelling tickets of those who are reselling wherever possible."
Ticketmaster said demand was "always going to be extremely high", with more than 450,000 searches for the event on its site in 24 hours.
Around 140,000 fans tried to get their hands on the £40 tickets.
Joel Hernon, who managed to get some, said he was "so honoured to have the opportunity to celebrate Manchester's strength on Sunday".
Another customer, Karl, said: "Over the moon to be attending One Love Manchester on Sunday and not forgetting that we are supporting those affected."
In a statement on Twitter last week, referring to the attack, Grande wrote: "We won't let this divide us. We won't let hate win."