Detectives have made "immense" progress in the investigation into the Manchester bombing and are confident they have arrested some "key players", Britain's top counter-terror officer has said.

Assistant Commissioner Mark Rowley said police have made "significant" arrests and "finds" and have got hold of a "large part of the network" being sought over the atrocity.

Mr Rowley said: "They are very significant, these arrests.

"We are very happy we've got our hands around some of the key players that we are concerned about but there's still a little bit more to do."

Mr Rowley disclosed that police have reviewed security at more than 1,300 events around the country ahead of the bank holiday weekend.

Britain remains at the highest threat level of critical, meaning an attack could be imminent.

READ: May raises UK's terror alert to 'critical'

The senior officer issued a message of reassurance to the public.

"Go out as you planned. Enjoy yourselves and be reassured by the greater policing presence you will see.

"We can't let the terrorists win by dissuading us from going about our normal business."

Twenty-two people were killed when Salman Abedi launched a suicide attack at an Ariana Grande concert at Manchester Arena on Monday night.

It was the worst terrorist atrocity to hit Britain since the July 7 attacks in London in 2005 and sparked a huge counter-terror probe.

Since Tuesday a total of 10 people have been arrested, with eight remaining in custody.

Two were released without charge.

Providing an update on the probe, Mr Rowley said it was likely further arrests would follow.

FACTBOX: What we know about Manchester attacker Abedi 

He said: "Having made enormous progress and made some significant arrests and had some significant finds, there still remain important lines of inquiry for us to pursue.

"We've got to try to understand everything we can about the dead terrorist, his associates.

Messages and tokens are left in tribute to the victims of the attack on Manchester Arena, in central Manchester. Photo: Reuters/Stefan WermuthMessages and tokens are left in tribute to the victims of the attack on Manchester Arena, in central Manchester. Photo: Reuters/Stefan Wermuth

"We need to understand the whole network and how they acquired and built the bomb that exploded on Monday night.

"It's going to take a little more time to close down those gaps in our understanding.

"We are working as fast as we can do because everyone wants answers to this."

He said police now have a "much better understanding" of what happened.

Mr Rowley went on: "We've got a lot of the risk contained but we still have some uncertainty.

"As we chase down those lines of inquiry we will keep this strong policing presence.

"We need to grow our confidence that we have got every component of the network and we have got as full an understanding as possible about how the device was constructed and whether there's any remaining risk.

"Clearly we haven't covered all the territory we want to but we have covered a large part of it.

"Our confidence has been increasing over recent days but there's still more to do to get to the degree of confidence we want."

Mr Rowley outlined a number of strands of the investigation into the attack:

  • Finances:  Officers are mounting a large-scale financial investigation, with Mr Rowley saying: "We are interested in equipment people have bought bearing in mind a bomb was constructed."
  • Communications: Police are looking back at communications and working closely with technology firms.
  • Exhibits: Investigators have seized large volumes of material during searches of a number of properties and expect to amass hundreds of electronic devices.
  • CCTV: Police are trawling through footage on local cameras to establish movements and patterns of activity in the lead-up to the attack.
  • Witnesses: Officers are working to "triage" hundreds of statements from those caught up in the bombing.
  • International inquiries: Detectives are probing links to Libya as well as liaising with counterparts across Europe and elsewhere.

Commenting on the progress of the investigation, Greater Manchester Police Chief Constable Ian Hopkins said 12 locations had been searched and thousands of items examined.

Speaking outside force HQ, he said: "I think it is fair to say it has been an extremely challenging week, and we are still in the middle of a live investigation.

"There are eight individuals in custody, they're aged between 18 and 38 years of age, and there are still 12 locations across Greater Manchester being searched, and that activity will continue."

Mr Hopkins added: "We have seized thousands of exhibits which are now being assessed. I think it is fair to say that there has been enormous progress with the investigation, but still an awful lot of work to do."

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