Video report by ITV News Correspondent Dan Rivers
A spate of attacks on a weekend of violence in London led to criticism of Sadiq Khan from Donald Trump, who called the London mayor a "disaster".
Retweeting a post by commentator Katie Hopkins about the killings, the president said the capital needed a new mayor "ASAP".
The original post by Hopkins called the capital "Stab-City" and "Khan's Londonistan" alongside two screenshots of 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."
A City Hall spokesman said Mr Khan was focusing on supporting communities and he was "not going to waste his time" responding to the president's tweet.
Labour leader Jeremy Corbyn responded to President Trump's tweet, accusing him of "using the tragedy of people being murdered" to attack Mr Khan.
On Katie Hopkins and Trump, he added: "They seek to divide at a time we need to come together."
What happened in London?
Two teenagers have been charged with the murder of an 18-year-old man who was stabbed to death in London during a weekend which saw three men killed in separate attacks in less than 24 hours.
A 17-year-old, who cannot be named for legal reasons, and Mohammed Nadir Dafallah, 18, from Wandsworth, were charged on Sunday.
They will appear in custody at Wimbledon Magistrates' Court on Monday June 17.
Emergency services were called to Deeside Road in Wandsworth, south-west London, at 4.42pm on Friday afternoon where the first victim was found stabbed.
He died at the scene and is yet to be formally identified, with a post-mortem examination due to be held.
In a second incident in a different part of London just minutes later, a 19-year-old man was fatally shot in Plumstead.
Mr Khan said he was "sickened" following the deaths of the two teenage boys.
On Saturday afternoon a man in his 30s was stabbed to death in Tower Hamlets, east London.
Three men were also stabbed in a separate attack in Clapham, south-west London, during a weekend of violence in the capital.
Mr Khan said: “I am sickened to hear that two young lives have been ended within minutes of each other in Wandsworth and Greenwich.
“Our overstretched police are working around the clock to keep Londoners safe. They need our support to end this scourge of violence.
“Anybody who has information about what happened either in Wandsworth or Greenwich should do the right thing and call the police.”