Four men are in police custody after being arrested in London as part of a pre-planned anti-terror operation.
Counter-terrorism police detained the men at 7.10pm on Sunday during raids on addresses across the capital.
A statement from the Metropolitan Police said the men were arrested under the Terrorism Act 2000.
Two 25-year-old men were detained in Mansell Street, east London. A 28-year-old man was arrested in Westbourne Grove, west London.
The fourth man - aged 29 - was detained in Peckham Street, southeast London.
Officers conducted a search of six addresses as part of the pre-planned operation.
"These searches are ongoing. Two vehicles are also being searched," said a police spokesman.
"Officers from the MPS Firearms Unit were involved in these arrests. Public safety remains our overriding concern."
A man who witnessed the arrest on Westbourne Grove - a street lined with restaurants and shops in Notting Hill - said he saw two police vans and a police car at the scene.
He said: "There was quite a lot of police. There was quite a scene as I approached."
Sky's defence correspondent Alistair Bunkall described the operation as "significant".
"Sometimes these arrests are made as a pre-emptive measure very early on in any planning for a terrorism operation, and sometimes they are made because the feeling is a terrorist attack may be imminent, but we haven't been given those sort of details yet," he said.
"But S015 officers, from Counter Terrorism Command, and armed Met Police officers were involved, so it certainly was a big operation by the Met."