We’re told the investigation is extreme right-wing terrorism-related.
It is connected to a suspected threat directed towards an Islamic event held at Shrubland Hall in Suffolk over the weekend.
Police say advice was provided to the organisers of the UK Ijtima event to close slightly earlier than planned yesterday as a result.
There is not believed be any wider threat to the public.
Eight people were held under the Terrorism Act and three were being questioned about claims of conspiracy to murder.
A woman was arrested in East London on suspicion of assisting an offender.
Commander Helen Flanagan, Head of Counter Terrorism Policing London said: “After becoming aware of a potential serious threat towards the Islamic event in Suffolk, we have moved extremely quickly to make a number of arrests in various locations across the country. I would like to thank colleagues in Suffolk Police for their ongoing support, which led to the declaration of a major incident on Sunday in response to this. I’d also like to thank the event organisers and the public who have been impacted by this.
“I know this news may be concerning to the public and particularly those in the Muslim community, given that we believe the intended target was an Islamic event. But as we have shown, we will not hesitate to act if there is any potential threat, no matter who or what the potential target may be. I’m also pleased to say that although the event ended slightly earlier than planned, it nevertheless passed off safely.
“Sadly, this activity is a stark reminder that the threat level in the UK is at ‘severe’, so we urge the public to remain vigilant and report anything if it doesn’t look or feel right.”
