Khan had told Londoners there was "no reason to be alarmed" about an increased police presence in the coming days.
In another message, Trump took a swipe at the courts and said the USA needs to a better job of screening people coming into the country because of concern about terrorism: "In any event we are EXTREME VETTING people coming into the U.S.in order to help keep our country safe".
Mr. Trump tweeted Monday morning that London Mayor Sadiq Khan had offered a "pathetic excuse" and "had to think fast on his "no reason to be alarmed" statement". "MSM (mainstream media) is working hard to sell it!"
"I invite all Londoners - and everyone visiting our city - to come together in solidarity to remember those who have lost their lives, to express sympathy with their families and loved ones and to show the world that we stand united in the face of those who seek to harm us and our way of life", Khan said.
Seven people were killed and almost 50 were injured in a terror attack involving a vehicle striking pedestrians on London Bridge and three attackers stabbing people in nearby Borough Market, authorities said.
US lawmakers from both political parties criticized Trump for raising the travel ban and assailing Khan.
The ex-LBC radio host has appeared on the Fox News channel in the wake of the attack on Saturday night, on a programme that Trump is known to watch religiously. "I think, again, the president's point is something he said frankly back nearly two years now - a year and a half ago - when the president talked about how we have to be more committed to national security".
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Sadiq Khan: London police should look into 'what they knew'
The Met have set up a casualty bureau on 0800 096 1233 and 020 7158 0197 for people concerned about friends or relatives. We will defeat you . "You do not commit these disgusting acts in my name", he said to applause at a vigil on Monday.
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On Sunday, a spokesman for the London mayor said he was working with the police, emergency services and government to coordinate the response to the attack, and had "more important things to do" than to respond to Trump.
President Trump also appeared to use the attack to promote his attempted travel ban executive order - and he continued on the same tack today.
"The point is, there is a reason to be alarmed".
It was met with a barrage of criticism in both Britain and the United States, with former Vice President Al Gore among many public figures who accused Trump of misrepresenting what Khan had said.
Khan, a human rights lawyer and practicing Muslim whose parents are from Pakistan, has repeatedly challenged Trump's calls to ban Muslims or people from majority-Muslim countries from entering the United States, saying the president has an "ignorant view of Islam".
"We are united, as I said, in our resolve, even against an enemy that thinks by hurting us they can scare us", Mr Mattis said.
Trump Jr. wrote "You have got to be kidding me?!" above a link to an article reporting comments Khan had made months earlier. You stand with them in times of adversity, but you call them out when they are wrong.