What happened: A vehicle mounted the sidewalk near the South Kensington museum at around 2:20 p.m. local time, hitting several pedestrians.
She further said, " I could see a crowd of people around what was clearly one or two people on the pavement", she said, confirming there were armed police at the scene.
Social media photographs show police and bystanders holding an injured man on the ground, while the road is sealed off with police tape.
Police have launched an inquiry to trace the motive behind the incident.
A spokesman later said the man had not been formally arrested, but had been detained.
Nearby Cromwell Road, a major thoroughfare, was shut down by police, causing heavy traffic congestion in parts of central London.
Authorities in London detained a man after he drove his vehicle through a group of pedestrians near London's Natural History Museum, according to officials.
After the incident, witnesses reported a auto hitting pedestrians.
Another user posted: 'Looks like a guy tried to mow people down at the natural history museum'.
A Downing Street spokesperson said British Prime Minister Theresa May is "being kept up to date on events". London police have also not said if this incident is related to terrorism.
The area of South Kensington in which the collision occurred is home to a number of museums, including the Natural History and Science Museum. "If you have any information about this incident, please call 101", police said in a statement.
The UK is on its second highest security alert level. In March, an assassin on Westminster Bridge in London was purposefully snapped into pedestrians, before he stabbed a policeman on site of Parliament.