By Alexis Akwagyiram
The police operation launched following the London terror attacks is the biggest the UK has seen, its impact felt across the country.
There is high-visibility policing on the public transport network
After bombs killed 56 people in a series of attacks on 7 July, the Metropolitan Police dramatically increased its presence on the capital's streets.
The failed attacks that followed two weeks later saw the operation stepped up further still.
Metropolitan Police Commissioner Sir Ian Blair described it as "the largest ever investigation that the Met has ever mounted".
That operation led to the arrest of Yasin Hassan Omar in Birmingham on Wednesday and the arrests of Muktar Said Ibrahim and Ramzi Mohammed in west London on Friday.
Osman Hussain, who is also named as Isaac Hamdi, was detained by Italian officers in Rome later on Friday. His extradition to the UK is being sought.
But the police investigation continues. Sir Ian has warned that there may be "other cells who are capable and intent on striking again".
The vast police operation has prompted a similarly strong public response, with 5,000 calls received by the anti-terror hotline.
Police have taken 1,800 witness statements and are looking at 15,000 CCTV tapes.
And while London remains the focal point of the operation, it extends far beyond the capital.
"This is a national enterprise in which the Metropolitan Police is the lead", said Sir Ian, adding that there has been "mutual aid from a number of forces".
Investigations are on-going in West Midlands, West Yorkshire, in Luton and Aylesbury, he said.
Police say 20 people have been held in custody as part of investigations - with arrests in Birmingham and Lincolnshire, as well as those in London.
It is also clear the operation is also being carried out on an international basis.
A spokeswoman for the Association of Chief Police Officers (ACPO) confirmed that police in the UK are liaising with "Interpol and all other foreign and international agencies", although she would not divulge any more details for security reasons.
Attempts to prevent further attacks on the public transport network have seen the British Transport Police's "biggest ever deployment" on the railways.
Sir Ian has warned there could be more attacks
Drawing attention to the existence of a threat outside London, it said its "high visibility operation" was in place "around the country".
This dual importance of both protecting and reassuring members of the public was echoed by ACPO.
"Alongside the operational response all forces are working closely with their local communities to provide reassurance and monitor and respond to any incidents," a spokeswoman said.
And, indicating the public's response to the current climate, she said "particular communities show increased anxiety" and had reported incidents of "abuse and threats in the street, paint daubing and minor criminal damage".
"Police are responding robustly to any such incidents but overall we assess that communities are relatively calm and we are optimistic that this may continue."