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Four men have been arrested in London as part of an investigation into Islamist-related terrorism, Scotland Yard has said.

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

The four men, aged 20 and 21, were arrested on suspicion of being involved in the commission, preparation or instigation of terrorism.

Met Commissioner Sir Bernard Hogan-Howe said it was a "quite serious case".

BBC security correspondent Frank Gardner said Whitehall officials were unable to comment on the nature of the potential plot they say may have been foiled.

Officials said the investigation was in its early stages and interviews and searches were ongoing, our correspondent added.

'Drumbeat around terrorism'

Those arrested were taken to police stations in central London and remain in custody. Officials said one individual had spent time in Syria.

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.

Speaking to BBC London, Mr Hogan-Howe said police were having to be more proactive around terrorism, which he said was being linked to conflicts in Iraq and Syria.

"It is difficult to talk in a great deal of detail about it, but it is a quite serious case and it's one of a series of arrests that we've had over the last few weeks, which taken together for me confirm that the drumbeat around terrorism has changed.

"It's a more intense drumbeat. We're having to be more interventionist and a lot of it is linked back to Syria and Iraq," he said.