Video report by ITV News Correspondent Sejal Karia
Tom Gray, a tour guide who was in the capital on business with his friend Stevie, was one of the brave members of the public who put their lives in danger to save others.
The attacker was a convicted terrorist who previously plotted to blow up the London Stock Exchange.
Speaking to ITV News, Mr Gray said he was "driving northbound over London Bridge" when the double decker bus he was behind stopped.
"We noticed several people jumping from the left side of the bridge into the central reservation, sort of out to the right.
"We just peered our heads around and sort of saw that there was one chap running away from about five guys running him down with a fire extinguisher.
"So Stevie and I just thought what to do and just kind of ran towards it, left the cars where they were, and then tried to do our best to apprehend the suspect."
He said the attacker was "wielding two knives, one was sort of duct taped to his hand".
"Someone sort of kicked the knife away, sort of northbound up London Bridge, and then after that, the police armed response were really, really quick, got there almost instantaneously."
He said officers then ushered him and his friend away from the area after being told the suspect was wearing a "bomb vest".
"We saw the guy get shot a couple of times and then hit the deck."
Thinking more of the safety of others than his own, Mr Gray protected "a school bus full of little children and made sure that the girls at the back were OK".
He added: "I wouldn’t say I kicked the knife, I kind of stamped on his wrist as hard as I could.
"And the the knife freed itself, and then somebody else sort of kicked the knife away."
Prime Minister Boris Johnson has praised the brave actions of heroes like Mr Gray as he paid tribute to "the extraordinary bravery of those members of the public who physically intervened to protect the lives of others and for me they represent the very best of our country and I thank them on behalf of all of our country".
Leaders of the three main political parties have suspended campaigning on Saturday in light of the attack.
The Metropolitan Police Commissioner, Cressida Dick, said the investigation into the terror attack is "very fast moving and dynamic", with officers working to identify the attacker and his motive.