Four men arrested, tasered on vague "suspicion" they "may have" foiled the "early stages of" a plan to attack the UK.

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Whitehall officials told the BBC the arrests "may have foiled the early stages" of a plan to attack the UK.

The men - two aged 21 and two aged 20 - were held by counter-terrorism police.

They were arrested on suspicion of being concerned in the commission, preparation or instigation of acts of terrorism, Scotland Yard said.

Officials said one individual had spent time in Syria. There was no word on alleged targets or how far advanced the alleged plot was.

Those arrested were taken to police stations in central London and remain in custody.

A number of residential addresses and vehicles are continuing to be searched by specialist officers in west and central London as part of the investigation.

Firearms officers assisted in one of the arrests but no shots were fired.

A 21-year-old who was Tasered by officers was not injured and did not require medical treatment, police said.