On 4 June 2017, President Donald Trump tweeted in response to comments made by Sadiq Khan, the Mayor of London, after a terrorist attack that hit the city the night before left at least seven people dead in the British capital:
At least 7 dead and 48 wounded in terror attack and Mayor of London says there is "no reason to be alarmed!"
— Donald J. Trump (@realDonaldTrump) June 4, 2017
President Trump appeared to have been responding to remarks made by Khan on Sky News, when in which the London mayor advised the public not to be worried if they saw more police on the streets of in the city in the days following the attack, saying: "Londoners will see an increased police presence today and over the course of the next few days. No reason to be alarmed — one of the things the police and all of us need to do is make sure we're as safe as we possibly can be."
The relevant section of Khan's remarks is captured the in video below starting at the 3.04 mark:
— Sky News (@SkyNews) June 4, 2017
Mayor Sadiq Khan was attempting to reassure the people of London that a greater police presence they might see in the aftermath of the previous day's attack was a public safety precaution; he was not indicating that another attack was imminent or that citizens should "not be alarmed" by what had occurred.