In the speech in Dallas, Texas, Mr Trump referred to reading a story "that in London, which has unbelievably tough gun laws, a once very prestigious hospital right in the middle is like a war zone for frightful stabbing wounds".
"They were brutally killed by a small group of terrorists that had guns".
During the speech he did not name the hospital or cite any evidence for his claims.
"Yes, that's right, they don't have guns, they have knives - and instead.is blood all over the floors of this hospital".
"London hasn't been used to that".
Dr. Karim Brohi, another Royal London surgeon, said that while it is true that there is knife violence in London which must be addressed "urgently and at all levels", no potential solution involves the use of guns.
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"We routinely have children under our care - 13, 14 15-year-olds are daily occurrences..."
Trump is due to visit the United Kingdom on July 13, after previously cancelling a trip amid calls for mass protests on the streets of Britain.
London Mayor Sadiq Khan's office declined to comment to CNN following Trump's remarks.
At least 38 people in London have lost their lives to knife crime so far this year as the city saw a spike in stabbings, according to Scotland Yard.
Trump didn't mention a specific hospital, but a surgeon at Royal London Hospital did say something very similar. Murders in the city rose to 153 in the year to the end of March, from 101 the previous year, according to the Metropolitan Police. In 2017, London's murder rate per 100,000 people was 1.2.
During his NRA speech, Trump also took aim at Chicago, saying it has tough gun laws but a problem with gun violence.
"They don't have guns".
Trump has a history of criticizing London, and has had well-documented spats with its mayor. The trip would conclude months of back-and-forth over when he'll visit America's closest ally. In January, he said he had "cancelled" a trip to formally open the new USA embassy in London because he thought the building was "a bad deal".
The US ambassador to Britain, Woody Johnson, said Mr Trump would "definitely" be coming to London, despite the warnings of likely demonstrations.