The father and two brothers of suspected suicide bomber Salman Abedi are among those detained.
His older brother, Ismail, was arrested in Chorlton, south of Manchester.
The father, Ramadan, and younger brother Hashem were arrested in Tripoli, Libya.
Witnesses say Ramadan Abedi was apprehended by masked men, believed to be Libyan forces, and whisked away.
In the moments before, he had given a TV interview in which he said he was certain his son was not responsible.
"Salman doesn't belong to any organisation, and I know that he doesn't belong to any. There was nothing hidden, because I had open discussions with him about anything he, Salman, wanted to understand. I'm sure and I'm certain that Salman didn't carry out such an act, but there are hidden hands behind this."
UK officials say 22 year-old Salman Abedi was known to security services, and probably did not act alone.
British-born Abedi appears to have been in Libya earlier this year, and only very recently returned to the UK.
Hamid El-Said is a British radicalisation analyst and says Salman Abedi's parents had tried to help their son.
"I spoke to a close family friend yesterday and he literally told me his parents were trying very, very hard to actually bring him back on the right path and they failed to do that and eventually he was doing very bad at his university, at his education. He didn't complete and they tried to take him back to Libya several times. He had difficulties adjusting to (a) European lifestyle."
Graphic details of the attack and its aftermath have been leaked by sources from the United States.
The New York Times has published photographs of the bomb's fragments, reporting it was probably packed inside a metal case inside a rucksack.
It reports the explosion was detonated by a handheld device, suggesting the victims were all killed in a circle around Abedi.
British home secretary Amber Rudd has said she was "irritated" by the leak.
Lord Carlile, a former independent reviewer of terrorism legislation in the UK, is also furious.
"I think the New York Times is being completely irresponsible in publishing this material. It's very, very annoying and irritating, I share Amber Rudd's irritation. There have been occasions in the past where the Americans have complained bitterly about information being known allegedly through British authorities."
Following the blast, Britain's terror threat level was raised to its highest - critical.
Nearly 900 soldiers have now been deployed to positions around parliament and key government buildings.
They are replacing armed police normally responsible for such tasks, allowing those officers to be deployed elsewhere.
London's Metropolitan Police Commissioner Cressida Dick explains.
"There are a number of unknowns since the attack - a huge number of inquiries. The officers in the counter-terrorism network near Manchester and beyond are working incredibly fast and whilst we're at 'critical' and there is the possibility of a further attack we want to be able to support the public as best we can and to protect them as best we can and we believe we need more armed officers on the streets."
London Mayor Sadiq Khan is urging people to not be deterred, and to go about their daily lives.
"People will see additional police officers, they will see officers who are carrying firearms and they will see military personnel. No reason at all to be panicked, no reason at all to be alarmed. We are doing all that we can to keep Londoners and visitors safe. That's the right course of action, to act on intelligence from the experts and they've raised the threat levels from severe to critical which is the highest form there can be."
Nearly two dozen people are severely injured and in a Manchester hospital, with several others still missing.
Australia's Attorney-General George Brandis says no victims were Australian.
"And we can confirm there were no Australians among the dead, and so far as we are aware there are no Australians among those who are injured."
