Police are given an extra seven days to question four men, aged 19 to 27, arrested in a series of raids in London and High Wycombe on Thursday and Friday.
Counter-terrorism police arrested a 19-year-old man in High Wycombe and a 22-year-old in Hounslow, west London, at 8.45pm on Thursday.
Armed police attended both arrests but no shots were fired.
A 27-year-old was arrested in a car in the street in Southall at 8.30pm on Thursday and a 25-year-old was arrested in Uxbridge, west London, at 2.55am on Friday morning.
Police have now been given an extra seven days to question the suspects.
All the men were taken to police stations in central London and remain in custody, while residential addresses and vehicles are being searched.
Police vehicles and tape could be seen outside two addresses in High Wycombe as officers carried out searches of the properties.
The arrests and searches are part of an “ongoing investigation into Islamist-related terrorism”, the Metropolitan Police said.
It is understood the men were believed to be plotting an attack on British soil.
The arrests come a few months after the UK terror threat level in the UK was raised from substantial to severe, meaning a terrorist attack is “highly likely”.
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.