Four people have been arrested across London in a pre-planned terrorism operation, Scotland Yard said.
A man who witnessed one of the arrests on Westbourne Grove - a street lined with restaurants and boutiques in Notting Hill - said what unfolded was "quite a scene".
The man, who did not want to give his name, said: "Well, I walked by and it seemed to be a couple of guys being arrested.
"There were three."
The man said the men "looked white" and were "up against the wall".