British police said on Monday they had arrested 12 people after a suspected threat directed towards an Islamic event at ​the weekend, as part of an investigation which they described ‌as “extreme right-wing terrorism-related”.

Police said they had made the arrests on Sunday and Monday at addresses across England after becoming aware of a “potential serious threat” ​towards the religious event.

The event, attended by about ​15,000 people, was held at a country house in Suffolk, ⁠eastern England, from July 9 to 12 and mostly went ​ahead as expected, apart from closing earlier than planned on Sunday.

​Interior minister Shabana Mahmood said she knew the news would be concerning for British Muslims.

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“We must stand against hatred and we must unite around our ​shared belief in a country that is open, generous and ​tolerant to all our communities,” she said in a post on X.

Britain ‌raised ⁠its national terrorism threat level to “severe” in April, signalling that a terrorist attack was considered highly likely, and counter-terrorism police head Helen Flanagan said the arrests on Monday were a “stark reminder” of ​that threat level.

​Eight men ⁠remain in police custody having been detained under the Terrorism Act, the statement said, while three ​other men were arrested on suspicion of conspiracy to ​murder, ⁠and a woman was arrested on suspicion of assisting an offender.

Those arrested were aged between 27 and 60 years old. A ⁠number of ​addresses in London and the surrounding ​areas, and in eastern England and near Manchester, are being searched.