Donald Trump has called Sadiq Khan "a disaster" in response to violence in London, where three men were killed in separate attacks in less than 24 hours.
Two teenagers were murdered within minutes of each other in different parts of the capital on Friday, while a man was stabbed to death on Saturday afternoon.
Retweeting a post by right-wing commentator Katie Hopkins about the killings, the president said the capital needed a new mayor "ASAP".
Mr Trump tweeted: "LONDON needs a new mayor ASAP.
"Khan is a disaster - will only get worse!"
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
The original post by Hopkins called the capital "Stab-City" and "Khan's Londonistan" alongside two screenshots of BBC News articles detailing the violence.
She wrote: "20 hours in Stab-City UPDATE 2 stabbed to death 1 shot dead Three stabbed - but not dead. Wandsworth & Tower Hamlets This is Khan's Londonistan."
Mr Trump later returned to Twitter, writing: "He is a national disgrace who is destroying the City of London!"
The president was responding to a Twitter user who said: "Khan is the reason I don't feel like visiting London anytime soon."
He is a national disgrace who is destroying the City of London! https://t.co/l3qcUS17jh— Donald J. Trump (@realDonaldTrump) June 15, 2019
While referring to the historic heart of the capital, which today serves as its financial and business centre, it is thought the president was talking about the Greater London area in his tweet.
Jeremy Corbyn said it was "absolutely awful to see @realDonaldTrump using the tragedy of people being murdered to attack the Mayor".
The Labour leader tweeted: "@SadiqKhan is rightly supporting the police to do their job while Katie Hopkins spreads hateful and divisive rhetoric.
"They seek to divide at a time we need to come together."
Absolutely awful to see @realDonaldTrump using the tragedy of people being murdered to attack the Mayor. @SadiqKhan is rightly supporting the police to do their job while Katie Hopkins spreads hateful and divisive rhetoric. They seek to divide at a time we need to come together. https://t.co/Se3fgWDXUZ— Jeremy Corbyn (@jeremycorbyn) June 15, 2019
A spokesman for the mayor said: "Sadiq is focusing on supporting London's communities and over-stretched emergency services.
"He has been in regular touch with senior Met police officers last night and throughout the day.
"His thoughts are with the victims' families. He is not going to waste his time responding to this sort of tweet."
The US president and London mayor have clashed in the past, including on Mr Trump's state visit at the start of June.
Mr Trump branded the mayor a "stone cold loser" in a two-part tweet as he touched down in Britain on June 3.
The Labour mayor then accused the president of "playground behaviour" and claimed he was a "poster boy" for the far-right.
An 18-year-old boy was stabbed to death in Wandsworth on Friday afternoon, minutes before another 19-year-old man was fatally shot in Plumstead.
Just hours later, a man in his 30s was killed in Tower Hamlets, east London.
Three men were also stabbed in a separate attack in Clapham during a weekend of violence in the capital.
The Metropolitan Police said 14 people - including several boys and a girl - have been arrested in connection with the incidents in Wandsworth, Plumstead and Clapham.