London, May 28 (IANS): The British intelligence has identified 23,000 jihadis extremists living in the country as potential terrorist attackers, the media reported on Sunday.

The scale of the challenge facing police and security services was disclosed by sources in Whitehall after criticism that multiple opportunities to stop the May 22 suicide bombing on the Manchester Arena had been missed, Xinhua news agency reported.

The bombing at the closing set of pop singer Ariana Grande's concert left

22 persons dead and 59 others injured.

About 3,000 of the 23,000 jihadis are judged to pose a threat, the report said.

Those 3,000 are under investigation or active monitoring in 500 operations conducted by police and intelligence services.

The rest have featured in previous inquiries and are categorised as posing a "residual risk".

The two terrorists who carried out attacks in Britain this year -- Manchester bomber Salman Abedi and Westminster attacker Khalid Masood, who killed five people two months ago -- were former subjects of interest and no longer under any surveillance, the report said.

Prime Minister Theresa May downgraded the terror threat in Britain from "critical" to "severe" on Saturday morning, but said troops will remain deployed on the streets until Monday.

Britain's top police officer for counter-terrorism said 11 men were held with more arrests expected.

London Metropolitan Police Assistant Commissioner Mark Rowley said significant arrests meant a large part of the terror network around suicide bomber Abedi had been wound up.

"The high pace and rapid progress of this investigation is continuing. There were three more arrests overnight and we now have 11 men in custody. There are now 17 searches either concluded or continuing at various addresses largely in the northwest of the country and we are getting a greater understanding of the preparation of the bomb," he said.