Khan had told London's citizens that there was no cause for alarm about increased police presence in the city in the coming days following an attack on the London Bridge that killed at least seven people.
Then, on Monday, he combined another shot at Khan with criticism of the mainstream media, tweeting: "Pathetic excuse by London Mayor Sadiq Khan who had to think fast on his "no reason to be alarmed" statement".
To huge applause, Khan told those gathered for the service in Potters Fields Park, near London's City Hall: "As a proud and patriotic British Muslim I say this you do not commit these disgusting acts in my name".
Earlier Monday, Prime Minister Theresa May said she thought Khan was doing "a great job", after President Trump derided and misrepresented the London mayor's attempt to calm Londoners after the attack. Khan also criticised Theresa May's decision to invite Donald Trump on a state visit, amid his continuing spat with the president. The organization, which supplies news reports to more than 6,700 news outlets and broadcasters, also found that Trump "got it wrong" in the Philippines case, calling it a terrorist attack when it was a robbery committed by a lone wolf.
Khan is among those who have asked the government to reconsider the invitation, describing the state visit as "inappropriate" given the president's months-long push for a travel ban for passengers from several Muslim majority nations.
Speaking on BBC Breakfast, Khan said: "Not unreasonably, these questions are being asked".
He accused Mr Khan of offering a "pathetic excuse" for saying there was no reason for people to be alarmed.
UK PM May says London incident treated as potential act of terrorism
President Donald Trump's spokesman says Trump has been briefed by his national security team and he will be provided with updates. But these last two to three months, we had two confirmed terrorist attacks, 22 people dying up in Manchester at the concert.
Forensics investigators work on London Bridge, after attackers rammed a hired van into pedestrians on London Bridge and stabbed others nearby killing and injuring people, in London, Britain June 4, 2017. The point is there is a reason to be alarmed. A spokesman for Khan responded that the mayor "has more important things to do than respond to Donald Trump's ill-informed tweet that deliberately takes out of context" his remarks.
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"One of the things about having close mates and having a special relationship is to not just stand with them in adversity but call them out when they are wrong".
Deputy White House spokeswoman Sarah Sanders told reporters on Monday that she did not think it was correct to characterise Trump's tweets as "picking a fight" with Khan.
Trump, however, doubled down on his attack Monday morning.
Trump's extraordinary attacks on the popular mayor of a grieving city have disgusted many Londoners, reigniting opposition to a state visit by the American president to the United Kingdom planned for later this year.