Mark Ringwood from Emsworth was in the capital this afternoon when a vehicle ploughed into pedestrians near the Natural History Museum which has left 11 people injured.
One witness said people were "fleeing for their lives" after the vehicle mounted the pavement.
"The people just panicked, panicking and running really, they didn't really know what was going on", he said.
The man pictured being restrained on the ground is understood to be the driver of the smashed-up Toyota.
But Scotland Yard later confirmed the incident, which occurred shortly after 2.20pm, was "not being treated as a terror-related incident" and a man had been arrested over a traffic collision.
Authorities determined that the incident was a "road-traffic collision" after four hours of investigation.
A man was detained at the scene, police said. The attack killed eight people and injured more than 40 others.
Video footages from the site have floated on social media which showed a man held down on the ground by the police following the accident. He spoke with deputy chief of Metropolitan Police, Mark Rowley, and continued to be in contact with him, told Khan via Twitter.
"When I heard the crash, I saw people running, I saw only the auto, it was panic". Some police came in and told us to get on the floor.
"They were pushing everyone forward and telling them to run".
Police at the scene on Cromwell Gardens in London, after several people have been injured after a auto reportedly ploughed into people outside London's Natural History Museum.
"A man...has been detained at the scene".
Armed police officers were deployed, with enquiries to establish the circumstances and motive undertaken.
It is the fourth most popular tourist attraction in the United Kingdom, with 4.6 million visits during 2016, according to the Association of Leading Visitor Attractions.
People reported being evacuated from the nearby Science Museum.