Terror arrests: Four men questioned over alleged plot
Police investigating an alleged Islamist terrorism plot targeting the UK are continuing to question four men arrested on Thursday and Friday.
The men were detained in west London and High Wycombe by counter-terrorism police.
The four, aged between 19 and 27, were all taken to police stations in central London after their arrests.
One official pointed out that the arrests came two days before Remembrance Sunday.
Scotland Yard has refused to comment on suggestions that the commemorations were the target of any plot.
Police said a 27-year-old man was arrested in a car in a street in Southall, at 20:30 on Thursday. Armed officers assisted but no shots were fired.
A 22-year-old man was arrested in Hounslow at 20:45, the same time that a 19-year-old was arrested in High Wycombe. Firearms officers assisted with both arrests.
A 25-year-old was arrested in Uxbridge at 02:55 on Friday.
They were all held on suspicion of being concerned in the commission, preparation or instigation of acts of terrorism.
By Danny Shaw, BBC home affairs correspondent
Last month, Scotland Yard warned that police were dealing with an "exceptionally high" number of investigations into Islamist-related terrorism - and so it's proving.
It's understood this investigation centres on an alleged plot to attack the UK. The arrests came just a few days before Remembrance Sunday and Armistice Day, heightening fears that people attending services may be at risk.
The fact that the arrests came at night, with one suspect detained in a car in the street, suggests there was some last-minute information or a trigger event that sparked police into action. Usually suspects are detained at dawn.
Did that information relate to events this weekend? At the moment that's speculation - police are giving little away.
Security adviser Dai Davies, a former head of royal protection, said while it was right for the authorities to take the threat of terrorist attack "very seriously", it was also important to keep a "sense of proportion".
Mr Davies, also a former Metropolitan Police chief superintendent, told the BBC: "I am sure, as far as it's humanly possible, they are doing everything they possibly can to ensure that everyone has a peaceful Remembrance service.
"It's so symbolic to the nation, particularly 100 years since the start of World War One, that we do remember the fallen heroes - and no-one, but no-one, should deter any of us from actually carrying out what is a lawful right to remember those who died on behalf of us."
The Metropolitan Police said a number of homes in Hounslow, High Wycombe, Uxbridge, Southall, Greenford and Hayes were being searched by specialist officers as part of the investigation.
In a statement, the force said the arrests and searches were "part of an ongoing investigation into Islamist related terrorism".
Officers from the Met's SO15 counter terrorism command are working with the south-east counter-terrorism unit and MI5.
The arrests come after the UK national terror threat level was raised from "substantial" to "severe" in August.
This means a terrorist attack is considered "highly likely".