‘I think he’s done a bad job on crime’

That’s not fair.

Mr. Khan assumed office only two years ago, replacing Boris Johnson, who went on to become Britain’s foreign secretary. London drew international attention this spring because of a spate of murders, mostly involving knives.

But the number of cases of “violence against the person,” which includes knife crime, has been growing steadily throughout this decade, and the general crime rate has been rising since a low point in 2014, according to London’s Metropolitan Police. All of this has occurred during an eight-year austerity drive by the national government that has squeezed budgets and reduced the number of police officers in London by 26 percent.

“London genuinely has a serious issue,” David Wilson, professor of criminology at Birmingham City University, said in a phone interview. “What we have to acknowledge is that there are issues in relation to keeping young people in school, giving them hopes for the future and tackling gangs and criminal networks which operate with young people at their heart.”

But Mr. Trump, he said, was “cherry-picking particular facts and figures not so much as a means to shed light on the phenomena but as a means to attack Khan, who has previously attacked him.”

“What we’re dealing with are long-term trends as opposed to single events,” he said, adding that he was particularly concerned about increasing numbers of young people joining organized crime gangs.