Media playback is unsupported on your device Media caption The BBC's Daniel Sandford: The arrests were due to "suspicions about a possible attack... with Remembrance Sunday the most immediate worry"

Police investigating an alleged Islamist terrorism plot targeting the UK have arrested four men.

Counter-terrorism police detained the men in west London and High Wycombe.

The men, aged between 19 and 27, were all taken to police stations in central London after their arrests overnight and remain in custody.

Police said one of the men was arrested in a car in a street in Southall, with armed officers assisting, but no shots were fired.

There is no current indication of a link to Syria or the activities of jihadist group Islamic State, BBC home affairs correspondent Danny Shaw said.

The arrest of a 27-year-old man in a car in Southall happened at 20:30 on Thursday.

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.

Analysis

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.

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".