"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.
At least seven people were killed and dozens injured after three terrorists drove a van into pedestrians on London Bridge and then got out to stab people.
This was in reference to Khan's statement that there was no cause for alarm. "You will see an increased police presence today, including armed officers and uniformed officers". That, he said, is "no reason to be alarmed", adding that "one of the things the".
Mr Khan led a vigil at Potters Fields Park paying tribute to the victims of the attack.
There was no immediate response from Khan to Trump's latest Tweet.
In an interview with ABC's "Good Morning America", Donald Trump Jr. said London's Mayor Sadiq Khan "should do something to fix the problem rather than just sit there and pretend there isn't one". "There's no reason to be alarmed". "And we, as a society, especially as Americans, better do something about it".
"He has risen above this crisis of death and destruction, as mayors continue to do, to alleviate fear, to bring comfort to his people of London", the mayors wrote in a statement on Sunday.More news: Trump Says Comic Should Be Ashamed for Severed Head Video
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"Sadiq Khan is doing a good job", May said, somewhat exasperated.
The attack raised security as a major theme of the general election, set for Thursday 8 June.
"This is a man insulting our national values at a time of introspection and mourning", Farron said on Twitter. "Londoners chant President Trump's name instead of mayor", and another was headlined "Sadiq Khan May Be In Trouble - Look What Londoners Are Doing!" Khan's comments followed up on calls he made last night.
Senior White House adviser Kellyanne Conway on Monday condemned what she called the media's "obsession with covering everything he says on Twitter and very little of what he does as president".
As Mr Trump was engaging in his attack on Mr Khan, the US Conference of Mayors, which represents 1,400 communities with populations of 30,000 or more, sent a message of support to the London mayor and to Londoners.
Trump is expected to visit the United Kingdom later this year.
He also used the attack to comment about U.S. gun rights.