Within an hour after three attackers drove a van into a crowd of people on London Bridge and then stabbed people in a nearby commercial area, Trump said, "We need to be smart, vigilant and tough".
That tweet drew angry rebukes from critics saying that the immediate aftermath of an attack that left 49 people dead was not a time for a president to be "basking in congratulations".
The attack comes days before the general election is due to take place, and after Britain's threat level was reduced back to "severe" after being raised to its highest level "critical" following the Manchester attack.
"The van mounted the pavement, and collided with pedestrians before being abandoned, where the attackers, armed with knives, continued into the Borough Market area, stabbing numerous people".
Earlier, the United States president also used the London attacks to promote and urge for support to his proposed travel ban on people from six Muslim-majority countries.
An armed police officer looks through his weapon on London Bridge in London, June 3, 2017.
The three attackers involved in the attacks, who were wearing hoax explosive vests, were shot and killed by police within eight minutes of the first call being made.
According to neighbours, the attacker had lived there for about three years and was married with two children.
Met Police: Assistant commissioner Mark Rowley arrives at Downing Street for a Cobra meeting.
Seven people were killed, police said. Oliver Dowling, 32, from New Zealand was stabbed in the face, but miraculously his sister announced he is expected to make a full recovery.
"One was particularly affected", the French foreign minister said on Sunday.
Seven people died and almost 50 were injured when a terrorist attack unfolded in the heart of the British capital on Saturday night.
Palestinians, Israel police clash at Jerusalem Old City gate
The International Christian Embassy in Jerusalem (ICEJ) echoed this sentiment in a strategy paper it delivered to Trump and U.S. Like Obama and many previous USA presidents, Trump has pressed Israel to curtail settlement construction in the West Bank .
In this tweet, Trump said, "At least 7 dead and 48 wounded in terror attack and Mayor of London says there is "no reason to be alarmed!".
Multiple witnesses reported a vehicle veering off the road and hitting several pedestrians; others also said they saw a man with a knife.
Trump's tweets have been heavily criticized by a number of British politicians, who are now preparing for the country's general election which is scheduled to take place on Thursday. "Defeating this ideology is one of the great challenges of our time".
Packed with people enjoying the summer night, the area around London Bridge and Borough Market soon turned to chaos and panic.
The Shard skyscraper - Britain's tallest building and one of the best-known sights on the London skyline - is also at the south end of London Bridge.
May's message: "We will take every measure available to us and provide every additional resource we can to the police and the security services as they work to protect the public".
The London Bridge area remains cordoned off and London Bridge station remains closed.
The death toll of seven didn't include the three attackers.
Pictures showed police leading members of the public away with their hands on their heads.
The assailant, Khalid Masood, 52, of West Midlands, reportedly had a criminal record and may have had connections to violent extremism, British Prime Minister Theresa May said.
Trump was criticized in some quarters for the remark, since it was unclear in-context whether Khan was addressing the attack or London's now-elevated police presence.
"Our understanding is growing and as we now understand it the van entered London Bridge at 9.58pm travelling from the north to the south side of the river".