Four men have been arrested on suspicion of terrorism offences in a series of raids, during which one suspect was Tasered.

The men - all aged 20 or 21 - were detained by officers from the Metropolitan Police's Counter Terrorism Command at addresses across London as part of an investigation into Islamist terror activities.

Officers believe the raids disrupted what could have been a "significant plot", according to a Whitehall source quoted by the Press Association.

The 21-year-old who was Tasered during his arrest did not require medical treatment, Scotland Yard said. 

"SO15 officers were assisted in the execution of warrants at one of the addresses by officers from the Specialist Firearms Command (SCO19). No shots were fired," police said in a statement.

"A number of residential addresses and vehicles are being searched by specialist officers in west and central London as part of the investigation. The searches are ongoing.

"These arrests and searches are part of an ongoing investigation into Islamist-related terrorism."

All four suspects have been taken to police stations in central London and remain in custody.