Four men have been arrested on suspicion of the commission, preparation or instigation of acts of terrorism, Scotland Yard said.
The men were arrested tonight under the Terrorism Act 2000 in a pre-planned intelligence-led operation in London involving armed officers.
They have all been taken to a south London police station, while six addresses across the capital are being searched, a force spokesman said.
The arrests took place at 7.10pm this evening, police said.
Two men were arrested in Mansell Street, east London.
One man was arrested at a premises in Westbourne Grove, west London.
The fourth man was arrested in Peckham Street, south-east London.
Police said public safety remains their overriding concern.
A man who witnessed the arrest 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".
He added: "And there was quite a lot of police. There was quite a scene as I approached."
He recalls seeing two police vans and a police car.
The area is now clear and there is no sign of an anti-terrorism operation having taken place, the man said.