British police have arrested a "large part of the network" behind the Manchester suicide attack but more arrests are likely, the country's top counter-terrorism officer said on Friday.
Mark Rowley said "immense" progress had been made in the investigation into Monday's suicide bombing that was carried out by a man police identified as Salman Abedi, a British national of Libyan origin.
Abedi blew himself up at a pop concert by US singer Ariana Grande in Manchester, killing 22 people, seven of them children, and injuring dozens others.
"They're very significant, these arrests. We're very happy we've got our hands around some of the key players that we are concerned about. But as I say, there is still a little bit more to do," Rowley told broadcasters.
Home Secretary Amber Rudd said the official threat risk remained at its highest level, "critical".
TRT World's Jon Brain has this report.
Hospitals have been warned to be ready.
However, Security Minister Ben Wallace said there was no evidence of a specific threat over Britain's holiday weekend, when major events will take place.
The FA Cup football final is set to be held on Saturday in London where extra armed officers will be on duty.
Nine people are being held by police following the bombing after the concert, including a man arrested on Friday evening. A further two people who were arrested earlier in the week have been released.
The campaigning for a national election on June 8 resumed after it was suspended following the attack.
The opposition Labour Party, emboldened by its rise in opinion polls, charged that Britain's foreign policy had increased the risk of attacks.
Labour leader Jeremy Corbyn also criticised Conservative Prime Minister Theresa May for cutting spending on policing.
In reaction May said: "I want to make one thing very clear to Jeremy Corbyn and to you, and it is that there can never, ever be an excuse for terrorism."
May was speaking to reporters at a summit of Group of Seven leaders in Sicily where she won support for action to prevent militants from using the internet to spread propaganda.
A new poll showed Labour had closed the gap with May's Conservatives to 5 points, suggesting a far tighter race than previously anticipated.