Four men were arrested in London on Wednesday on suspicion of terrorism offences. The arrests "may have foiled the early stages" of planning an attack in the U.K., British officials told BBC. They said that one of the suspects had spent time in Syria.

One of the arrested men was tasered during the series of raids on several addresses across London. He did not require treatment, Scotland Yard said.



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



"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."

The suspects remain in custody in central London police stations, and homes and vehicles in west and central London are being searched as part of what police called an ongoing investigation into Islamic-related terrorism.

British police arrested 11 people last month in other Islamist-related operations. It was not immediately clear if the various investigations were connected.

The terror threat in the U.K. was recently raised to "severe" because of the increase in the number of Britons traveling to Syria to join up with the Islamic State group. The government has said the militant group plans to strike targets inside Britain.