What happened: A vehicle mounted the sidewalk near the South Kensington museum at around 2:20 p.m. local time, hitting several pedestrians.
The incident left traffic gridlocked, but many tourists at the scene didn't seem concerned, he said.
"The incident is a road traffic investigation and not a terrorist-related incident".
In London, a vehicle near Natural History Museum in Kensington district has been driven to a sidewalk and has captured several passers-by.
The area, which includes the wide, semi-pedestrianised Exhibition Road which runs past the Science Museum and the Victoria & Albert Museum, is popular with families and tourists, particularly on a Saturday afternoon.
Downing Street said Prime Minister Theresa May was being kept up to date with developments.
Police are investigating the circumstances surrounding the incident and the driver's possible motive.
11 people have been treated for injuries sustained in the incident, with nine being taken to local hospitals. She said she was told by police injuries sustained were minor.
London Mayor Sadiq Khan tweeted that he was in close contact with Assistant Metropolitan Police Commissioner Mark Rowley, who plays a lead role in the police's counter-terrorism operation.
One man was detained at the scene.
A Reuters witness said there were large numbers of police, including armed officers, and paramedics at the scene although the atmosphere appeared calm.
Eyewitnesses said that police rushed into each bar and restaurant in the area and told people to get out.
The suspect, identified as a 52-year-old British man named Khalid Masood, also stabbed an unarmed police officer.
Eight people died in early June in an attack on London's nightlife District Borough Market and London Bridge.