After the Islamic State (ISIS) claimed the 2017 London Bridge terrorist attack, Mr. Trump said Mr. Khan was guilty of "pathetic" behaviour and in July previous year he claimed the Mayor had "done a very bad job on terrorism".
A spokesperson for City Hall was reported to comment that Mr Khan's thoughts were with the victim's families and that he was "not going to waste his time" replying to the President's latest attack.
Khan, London's first Muslim mayor, had led opposition to Trump's visit, writing a newspaper article in which he compared the United States leader to European dictators from the 1930s and 1940s.
"LONDON needs a new mayor ASAP", Trump posted.
At least 14 people have been arrested in connection with the attacks. "Khan is a disaster - will only get worse!" he wrote.
Mr Khan was focused on supporting London's communities and "over-stretched" emergency services, they added.
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He said Mr Khan was "rightly supporting the police to do their job while Katie Hopkins spreads hateful and divisive rhetoric".
On Katie Hopkins and Trump, he added: "They seek to divide at a time we need to come together".
Mr Trump's feud with Mr Khan dates back several years. "I can tell you, we got crime down by 20 per cent, we got the murder rate down", Mr Johnson said recently when launching his campaign.
The BBC also reports that in the early hours of Saturday, two men were stabbed in Clapham, south-west London, while another was stabbed in Brixton, south London.
Another three men were slashed in a separate knife attack at about 3am on Saturday.
The deaths take the total number of murders in London in 2019 to 56.