London Mayor Sadiq Khan has called Donald Trump a "poster boy for racists" after the US President hit out at him over London's knife crime.
Mr Trump retweeted a post from right-wing commentator Katie Hopkins blaming the violence on "Khan's Londonistan".
Her comments came after four people were killed in a spate of shootings and stabbings in London over three days.
Foreign Secretary and Tory leadership candidate Jeremy Hunt said he "150% agreed" with Mr Trump.
Speaking in central London on Monday, Mr Khan said: "It's remarkable that you've got the president of the USA amplifying the tweets of a far-right activist, amplifying a racist tweet.
"That's one of my concerns about Donald Trump - he's now seen as a poster boy for racists around the world, whether you're a racist in this country, whether you're a racist in Hungary, a racist in Italy, or a racist in France."
The original post by Ms Hopkins called the capital "Stab-City", alongside screenshots of BBC News articles detailing the violence.
But a number of people pointed out the much higher homicide rates in US cities.
LONDON needs a new mayor ASAP. Khan is a disaster - will only get worse! https://t.co/n7qKI3BbD2— Donald J. Trump (@realDonaldTrump) June 15, 2019
Mr Hunt said Mr Trump had his "own style", but he backed the president's stance on Mr Khan.
"We have a Mayor of London who has completely failed to tackle knife crime and has spent more time on politics than the actual business of making Londoners safer and in that I 150% agree with the president," he said while attending a Conservative leadership hustings event.
But Mr Khan said: "We've had four days and four homicides in London, we've seen over the last five years an increase in violent crime across our country and it's not acceptable.
"That's one of the reasons why City Hall, even though there's been massive cuts from central government, have continued to invest in our police."
He added: "There are many good leaders in America facing massive increases in violent crime.
"They have my support to make sure we learn lessons from each other and that we work together to grapple the issue of violent crime taking place in many cities across the Western world."
Another Tory leadership hopeful, Home Secretary Sajid Javid, said President Trump should be more worried about violent crime in the United States.
Mr Javid said: "I think President Trump should stick to domestic policies and I think it is unbecoming of a leader of such a great state to keep trying to interfere in other countries' domestic policies.
"The president is right to be concerned about serious violence, but he should be concerned about the serious violence in his own country where it is more than 10 times higher than it is in the UK."
Shadow Home Secretary Diane Abbott said she had never heard any previous US President make reference to a London mayor at all.
In the Commons, Ms Abbott said: "It's hard to escape the conclusion that President Trump may be singling out Sadiq Khan because he is of the Muslim faith."