British security forces worked Wednesday to wrap up a family based terrorist ring, as the father and brother of the British-born suicide bomber were arrested in the father's native Libya.
Fully armed and combat-ready UK soldiers were deployed on streets in London, Manchester and other cities in a show-of-force security operation.
The British government says a national minute of silence will be observed on Thursday to remember those who died or were affected by Monday's bombing at Manchester Arena.
The bombing killed 22 people and wounded scores at an Ariana Grande concert aimed at teen and tween girls. Another 59 people were wounded and hospitalised, and 60 others were treated and released.
The silence will take place at 11am (5pm Thursday, Thailand time). Officials also said that flags will remain at half-mast on government buildings until Thursday evening.
Britain's political parties say they plan to resume campaigning Friday for the June 8 general election Thursday, after putting campaign events on hold to show respect to bombing victims.
In Tripoli, a government spokesman confirmed an earlier eyewitness report that Abedi's father had been arrested by the Libyan counter-terrorism forces.
Special Deterrent Force spokesman Ahmed bin Salem saidRamadan Abedi, the father of Salman Abedi, was detained in Tripoli on Wednesday, "for interrogation".
Before his arrest, the father had told the media that his son was innocent and had been planning a trip to Saudi Arabia for a pilgrimage.
Salman Abedi, 22, the British-born suicide bomber. Security officials believe others masterminded his attack on Monday's Manchester concert that killed 22.
The father, Ramadan Abedi, also had said he worked as the administrative manager of the Central Security force in Tripoli.
He said he fled Tripoli in 1993 after Moammar Gadhafi's security authorities issued an arrest warrant and eventually sought political asylum in Britain.
While the suicide bomber Salman Abedi was born in Britain, his mother and emigrated from their native Libya before the family wound up in Manchester.
Two brothers of the bomber were arrested in the hours immediately following the suicide attack on the concert.
There have been six arrests, including a man literally taken down on the street in Wigan on Wednesday and a woman detained late Wednesday during an armed raid at a block of flats in Blackley, north Manchester.
The UK terror threat level is now at its highest level of "critical", meaning more attacks may be imminent.
The British Home Secretary, Amber Rudd, said the concert attack "was more sophisticated than some of the attacks we've seen before, and it seems likely - possible - that he wasn't doing this on his own."
The BBC's security correspondent Frank Gardner quoted security sources that it is believed the bomber, Salman, was likely only a "mule", using a device built by someone else in an operation probably ordered by still others.