The eldest son of President Donald Trump tweeted: "You have to be kidding me?!: Terror attacks are part of living in big city, says London Mayor Sadiq Khan," along with a link to an article by The Independent from September 2016.
The article, which was written after a bombing in New York City, quotes Khan as saying that terrorist attacks are "part and parcel of living in a big city".
The mayor said that the bombing gave him a "sleepless night" and noted that major cities "have got to be prepared for these sorts of things" to happen when least expected.
"That means being vigilant, having a police force that is in touch with communities, it means the security services being ready, but it also means exchanging ideas and best practice," Khan told the Evening Standard.
Pippa Crerar, the Evening Standard journalist who interviewed Khan, defended his remarks in her own Twitter message. "I was journo @SadiqKhan said this to after NYC attack. He was being realistic: cities must be prepared. Failure of l'ship to do otherwise," she wrote.
Trump Jr faced strong criticism from Wes Streeting, the MP for Ilford North, and Channel 4 News correspondent Ciaran Jenkins, The Guardian reported.
"You use a terrorist attack on our city to attack London's Mayor for your own political gain. You're a disgrace," Streeting wrote.
Jenkins tweeted: "Is this helpful @DonaldJTrumpJr? Did you even read the article before goading London's Mayor during a live incident?" He later added, "Headline is based on very first sentence, which if you'd bothered to read it could apply to any major city in the world. Key word: 'threat'."
In his statement following Wednesday's (22 March) attack, Khan said, "Londoners will never be cowed by terrorism."
"I want to reassure all Londoners and all our visitors not to be alarmed. Our city remains one of the safest in the world. London is the greatest city in the world. And we stand together in the face of those who seek to harm us and destroy our way of life. We always have and we always will," the mayor added.
Khan, the first Muslim mayor of London, is a vocal opponent of President Trump, The Hill noted. The mayor has called Trump "ignorant" and asked Prime Minister Theresa May to suspend a planned state visit by the president to the UK.