Donald Trump has been slammed for showing a "diminished ... mental capacity" after the president mistakenly referred to "Toledo" as the location of this weekend's Ohio mass shooting, which actually took place in Dayton.

Mr Trump's slip-up occurred on Monday morning during remarks from the White House in which he called on the United States to condemn "racism, bigotry and white supremacy" following two mass shootings that occurred over the weekend within a 13 hour span. At least nine people were killed in the shooting in Dayton, while at least 21 were killed in an earlier shooting in El Paso, Texas.

"May God bless the memory of those who perished in Toledo, may God protect them. May God protect all of those from Texas to Ohio. May God bless the victims and their families," Mr Trump said.

His remarks were quickly criticised by Ohio congressman and 2020 democratic presidential candidate Tim Ryan, whose congressional district is northeast of the site of the shooting.

"It's heartbreaking because he's showing diminished capacity, mental capacity, to be able to lead," Mr Ryan said during an appearance on CNN.

In a tweet just before that appearance, Mr Ryan was more succinct: "Toledo. Fck me", he wrote.

A transcript of Mr Trump's remarks provided to reporters following his speech shows that "Toledo" had been crossed out. It is not clear whether the city's name was crossed out before or after the president said it on live television. (Screenshot)

Soon after Mr Trump's mistake, the mayor of Dayton, Nan Whaley, ridiculed the president, and noted that she had not received a call from Mr Trump in spite of rumours that he planned to visit the city later this week.

"I've heard that he's coming Wednesday, but I have not gotten a call," she said. "And, you know, he might be going to Toledo. I don't know."

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Mr Trump was not the only septuagenarian political leader in America to mix up the names of the cities that had endured the mass shootings this weekend, with former vice president Joe Biden also making that mistake.

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During a fundraiser Sunday night in San Diego, Mr Biden — a front-runner for the democratic nomination — mistakenly called the shootings "the tragic events in Houston today, and also in Michigan the day before."