Speaking at a pro-gun rally in Dallas, the US president compared a London hospital to a “war zone” and said there was “blood all over the floors” because of the spate of fatal stabbings in the capital this year.
Mr Trump told the National Rifle Association (NRA) convention: “I recently read 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 horrible stabbing wounds.
“Yes that’s right, they don’t have guns, they have knives. And instead there’s blood all over the floors of this hospital. They say it’s as bad as a military war zone hospital.”
Making a stabbing motion with his hand, Mr Trump then said: “Knives, knives, knives.”
He added: “London hasn’t been used to that, they’re getting used to it. It’s pretty tough.”
It is not clear which specific story Mr Trump was referring to, but a trauma surgeon from the Royal London Hospital spoke out last month about the problems of both gun and knife crime.
Martin Griffiths said some of his military colleagues had likened working in the city to their time at Camp Bastion in Afghanistan.
Responding to Mr Trump, Professor Karim Brohi, a trauma surgeon at The Royal London Hospital, said: “Knife violence is a serious issue for London. We are proud of the excellent trauma care we provide and of our violence reduction programmes.
“The Royal London Hospital has cut the number of our young patients returning after further knife attacks from 45% to 1%.
“There is more we can all do to combat this violence, but to suggest guns are part of the solution is ridiculous.
“Gunshot wounds are at least twice as lethal as knife injuries and more difficult to repair. We are proud of our world-leading service and to serve the people of London.”
Mr Griffiths also responded to Mr Trump, saying he would be happy to invite the president to the Royal London Hospital to “discuss our successes in violence reduction in London”.
Mr Trump is due to visit the UK in July, although a trip to London has not been confirmed.
At least 38 people in London have lost their lives to knife crime so far this year, according to the Met Police.
During his speech, Mr Trump also turned his attention to the 2015 Paris terror attacks that left 130 people dead.
Making a gun gesture with his fingers and shouting “boom”, he told the audience that “if one employee or just one patron had a gun… or if one person in this room had been there with a gun” the attackers would have been stopped in their tracks.
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He said: “The terrorists would have fled or been shot and it would have been a whole different story.”
The president visited Dallas and urged NRA supporters to back Republican candidates in upcoming congressional elections, warning that Democrats would look to tighten gun laws after a series of mass shootings in the US.