Trauma surgeons in London, meanwhile, said Trump had missed the point when, in the same speech, he linked knife crime there to an absence of guns.
Comments by Trump have caused upset before in Britain. Relations with Prime Minister Theresa May cooled last year after she criticised him for retweeting anti-Islam videos by a British far-right group.
Trump, who is due to visit Britain on July 13, told NRA members that a “once very prestigious” London hospital, which he did not name, had become overwhelmed with knife attack victims.
“They don’t have guns. They have knives and instead there’s blood all over the floors of this hospital,” he said. “They say it’s as bad as a military war zone hospital. Knives, knives, knives, knives,” he said, making stabbing gestures.
London suffered a spike in knife crime early this year, and saw more murders during February and March than New York.
Last month, trauma surgeon Martin Griffiths told the BBC some of his colleagues had likened the Royal London Hospital in east London where he works to the former British military base Camp Bastion in Afghanistan.
But on Saturday he indicated Trump had drawn the wrong conclusion from his remarks.
Griffiths posted his comment next to an animation of a stick figure with the phrase “The Point” flying over its head and also linked to a statement on the hospital’s website by fellow trauma surgeon Karim Brohi.
“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,” Brohi said in the statement on Saturday.
Britain’s government effectively banned handgun ownership in England, Scotland and Wales after a school shooting in 1996.