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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".
The Metropolitan Police said searches are being carried out at six addresses after four people were arrested on suspicion of terror offences.
In a statement, Scotland Yard said: "At approximately 19.10 today (Sunday, 13 October) four men were arrested on suspicion of the commission, preparation or instigation of acts of terrorism under the Terrorism Act 2000.
"They have all been taken to a south London police station. Two men were arrested in Mansell Street, E1. One man was arrested at a premises in Westbourne Grove W2. The fourth man was arrested in Peckham Street, SE15.
"Searches under the Terrorism Act 2000 are being carried out at six addresses in London. Officers from the MPS Firearms Unit were involved in these arrests. Public safety remains our overriding concern."
Four people have been arrested across London in a pre-planned terrorism operation, the Metropolitan Police have said:
Latest ITV News reports
Detectives spent the day questioning four suspected Islamist terrorists amid fears the men were plotting to launch an attack in Britain.