British police have arrested four men in their early 20s suspected to have been planning a terrorist attack.

Tuesday's arrests came as police raided several locations across London.  No shots were fired but officers needed to use a Taser on one of the suspects, temporarily disabling him with an electric shock.

Authorities are still searching a number of houses and cars in London while they question the suspects - one of whom is alleged to have ties to Islamic militants in Syria.

Officials gave no details on the planned attack, but said it was in the early stages, describing it as significant and serious.

Britain raised its terrorist threat level to severe in August.

The Islamic State in Syria has beheaded two British aid workers and two U.S. journalists to avenge U.S.-led airstrikes on the Islamic State in Iraq.