Later, Trump spoke to British Prime Minister Theresa May and extended his support for America's closest ally. He tweeted, "Whatever the United States can do to help out in London and the UK, we will be there - WE ARE WITH YOU. GOD BLESS!"
Today, however, once the breadth of the horror in London was clear, Trump was back on Twitter. He criticised the city's mayor - Sadiq Khan, a liberal Muslim and an old Trump foil - for not being tough enough protecting his citizens.
"At least 7 dead and 48 wounded in terror attack and Mayor of London says there is 'no reason to be alarmed!'" Trump tweeted.
Trump took Khan's quote out of context. The Mayor had urged Londoners, in a BBC interview that was replayed, not to be "alarmed" by an increased police presence in the city. He said that after condemning the "deliberate and cowardly attack" as "barbaric".
A Khan spokesperson swatted away Trump's taunt, saying in a statement 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 urging Londoners not to be alarmed when they saw more police - including armed officers - on the streets."
Trump also stoked the long-running and emotionally-charged national debate over gun laws by pointing out that the London attackers did not use firearms. "Do you notice we are not having a gun debate right now? That's because they used knives and a truck!," Trump tweeted.
Britain has some of the world's strictest laws restricting gun purchases. The death toll in London might have been higher had the attackers used the kind of semiautomatic weapons that are more easily attainable in the United States.
White House officials did not respond to questions about Trump's response.
With Trump spending another day at his private golf club in suburban Sterling, Virginia, the White House's social media director, Dan Scavino, revived an old Trump-Khan feud on Twitter and scolded the mayor to "WAKE UP!!!!"
Chris Lu, who served as White House Cabinet secretary under President Barack Obama, was aghast.
"The fact that the White House social media director is commenting before the national security leadership has spoken is yet another example of Trump's 'shoot first, ask questions later' attitude towards handling international incidents," Lu said.
Historian Robert Dallek said Trump is exhibiting an entirely new style of presidential leadership. "Trump rubs everything raw," he said. "He makes it more acerbic, more contentious."
Dallek, who has studied former president Franklin D. Roosevelt, who steered the country through Pearl Harbour, was unsparing in his critique of Trump's response to the London attack.
"There's something so petty about this man," Dallek said. "What we're dealing with is someone who is, and I think this is the best term, an egomaniac. Everything has to revolve around him - he knows better, he's right, he one-ups everything."
Trump's supporters are likely to see his swift flurry of commentary as evidence of strength and unwavering resolve - a leader dispatching with political correctness and caution to deliver an assessment that is authentic and immediate.
This is just how Trump behaved on the campaign trail. He was quick to pounce on terrorist incidents in Paris and Brussels, as well as Orlando and San Bernardino, California, with tough vows, even if he was loose with his facts.
Last month, after a suicide bomber killed 22 others and injured scores more at an Ariana Grande concert in Manchester, England, Trump labelled terrorists "evil losers" and vowed to obliterate "this wicked ideology".
Trump last week also prematurely called a deadly attack in a casino in the Philippines a "terrorist attack". Philippine President Rodrigo Duterte later said it was not the work of terrorists but a "crazy" gunman.
Trump's response to this weekend's London Bridge incident won praise from friend Nigel Farage, who as head of the UK Independence Party led last year's Brexit movement, which Trump supported and saw as a precursor to his own election.
In an interview on Fox News Channel's Fox and Friends, a show Trump is known to watch frequently, Farage sharply criticised Khan and May's responses to the London attack as too timid and politically correct. He also lamented that the city had become, in his assessment, a safe harbour for Muslim "radicals".
"We don't just want speeches given outside 10 Downing Street," Farage said. "We want genuine action. And if there's not action, then the calls for internment will grow."
Trump echoed Farage's broad sentiment, assailing political correctness in the United States as well. "We must stop being politically correct and get down to the business of security for our people. If we don't get smart it will only get worse," Trump said on Twitter.
Although Trump and May have a relationship that both countries describe as positive and productive, Trump has long tangled with Khan, a member of the Labour Party who was elected mayor last year, London's first Muslim chief executive.
Khan has positioned himself as a moral and ideological foil to Trump. During last year's US presidential campaign, Trump proposed banning all Muslims from entering the United States, but suggested he would make an exception for London's mayor. Khan responded by saying Trump had an "ignorant view of Islam".
This January, Khan criticised Trump's travel ban on people from seven majority-Muslim countries - it was later revised to six. The mayor called it "shameful and cruel," saying that the policy "flies in the face of the values of freedom and tolerance".
And just last week, Khan joined the chorus of foreign leaders denouncing Trump's decision to withdraw the United States from the landmark Paris climate agreement.
In the aftermath of the London attack, Trump's critics chastised him for continuing his feud with Khan.
"I don't think that a major terrorist attack like this is the time to be divisive and to criticise a mayor who's trying to organize his city's response to this attack," former US Vice-President Al Gore said on CNN. "The terrorists want us to live in a state of constant fear."
