Britain's spies believe there may be as many as 23,000 terror suspects at large across the country.
The scale of the challenge has emerged in the aftermath of the Manchester suicide bombing that claimed 22 lives and injured 119 others.
Reports that Libyan-origin Salman Abedi had been on the radar of intelligence services have added pressure on MI5 to reveal what they knew.
Now, government sources have told sections of the UK media that they believe 23,000 people with extremist tendencies are living in the UK.
About 3,000 people from the total group are judged to pose a threat and are under investigation or active monitoring in 500 operations being run by police and intelligence services.
The 20,000 others have featured in previous enquiries and are categorised as posing a "residual risk".
Meanwhile, Greater Manchester Police have released CCTV images showing suicide bomber Abedi on the night he attacked Manchester Arena at the end of an Ariana Grande concert.
The images are the first to show what the 22-year-old looked like when he carried out the deadly terror attack.
Fourteen locations are still being searched in connection with the attack and 11 men remain in custody on suspicion of terrorism offences.
A Greater Manchester Police statement said: "We have made significant progress in this fast moving and complex investigation, working with the national counter-terrorism policing network and UK intelligence partners and securing the arrests of 11 people who remain in custody."
"The level of resources we have available to us remains the same as we continue to take positive action so you will notice additional officers, including armed officers, on patrol especially at a number of events this weekend to ensure the security and safety of everyone but it's important that people remain alert and vigilant," the statement added.
They have made an appeal to the public for information about the bomber's movements since May 18, when he returned to the UK from Libya.
A joint statement from Greater Manchester Police Chief Constable Ian Hopkins and Neil Basu, Scotland Yard's senior national co-ordinator from UK Counter Terrorism Policing, added: "This is still a live investigation which is not slowing down. Our priorities are to understand the run up to this terrible event and to understand if more people were involved in planning this attack."
The UK's terror threat level had been lowered to "severe" from "critical" on Saturday as a result of the arrests, which means an attack is still highly likely if not imminent.