Donald Trump Jr. tweeted about the London attack — and got instantly dragged

Politicians are often accused of trying to capitalise on terror attacks for their own agenda.

So when Donald Trump Jr. sent a contemptuous tweet criticising London Mayor Sadiq Khan in the aftermath of the Westminster terror attack that left three dead, Britons weren't happy.

The U.S. president's son tweeted a link to a September 2016 story in The Independent, quoting Khan as saying terror attacks were "part and parcel of living in a big city."

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He added: "You have to be kidding me?!: Terror attacks are part of living in big city, says London Mayor Sadiq Khan".

The interview was taken out of context and it's not clear whether Trump Jr understood that the quote was from six months ago and not from the Wednesday attack.

In the Independent article, Khan was speaking after a meeting with New York Mayor Bill De Blasio following the bombing in Chelsea, New York City, and nearby towns that wounded 29 people.

After revealing he had a "sleepless night" after the attacks, the London mayor said:

"It is a reality I'm afraid that London, New York, other major cities around the world have got to be prepared for these sorts of things."

"That means being vigilant, having a police force that is in touch with communities, it means the security services being ready, but also it means exchanging ideas and best practice."" he said.

People on Twitter were quick to slam Trump Jr. for the tweet, accusing him of exploiting the tragedy.

Wes Streeting, the MP for Ilford North and former president of the National Union of Students, tweeted at him:

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Ciaran Jenkins, Channel 4 News correspondent, said:

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Other people accused the president's son of trying to pass off an article written in September as if it was in reaction to the London attack:

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The mayor's actual response to the Westminster attack was that Londoners "will never be cowed by terrorism":

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“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 said.