Four men were arrested in connection with an alleged Islamist terror plot - just two days before Remembrance Sunday.
Counter-terror police detained the men, aged 19 to 27, on Thursday night at locations across west London and High Wycombe in the Thames Valley area, Scotland Yard said.
Police vehicles and tape could be seen outside two addresses in High Wycombe - in Desborough Avenue and West Wycombe Road - as officers carried out searches of the properties.
It is understood the men were believed to be plotting an attack on British soil. The timing of the arrests raised fears that Remembrance Sunday events in London were alleged targets.
A 19-year-old, understood to be called Yousaf Syed, was arrested at an address in High Wycombe at the same time as a 22-year-old was arrested in Hounslow at 8.45pm on Thursday. Firearms officers assisted with both arrests.
A 25-year-old was arrested in Uxbridge at 2.55am this morning while a 27-year-old was arrested in a car in the street in Southall at 8.30pm on Thursday.
Earlier this month, senior police officers and MPs said there was likely to be a significant rise in the number of armed police at the Remembrance Sunday and Armistice Day commemorations in London due to increased fears of a terror attack.
The arrests come a few months after the national terror threat level in the UK was raised from substantial to severe, meaning a terrorist attack is ''highly likely''. This assessment is separate from the evaluation concerning police officers.
The Joint Terrorism Analysis Centre (JTAC) raised the level against a backdrop of increasing concerns over hundreds of aspiring British jihadis travelling to Iraq and Syria to learn terrorist ''tradecraft'' and fight alongside terror groups such as Islamic State (IS).
Although there is currently no link to Syria or IS, fears were raised when IS fighters encouraged supporters in Western countries to "rise up" and commit acts of terror in their home countries.
Armed officers assisted at two premises and during an on-the-street arrest, the force said, although no shots were fired.
All the men were taken to police stations in central London and remain in custody, while a number of residential addresses and vehicles are being searched.
The arrests and searches are part of an "ongoing investigation into Islamist-related terrorism", the Metropolitan Police said.
Officers from the Metropolitan Police Service's Counter Terrorism Command (SO15) are working with the South East Counter Terrorism Unit (Sectu) and MI5.
No further details of the arrests were released.
It is understood at least one of the arrests in west London was made in the borough of Hounslow.