Police recovered several knives during the “intelligence-led” swoop.
Terror police are continuing to question seven people, including a suspected knifeman arrested in the heart of Westminster, held after separate operations in London.
The swoops on a 27-year-old man in Whitehall and locations in north London and Kent were executed to contain “threats” and an “active” terror plot, Scotland Yard said.
One suspect, named as Khalid Mohamed Omar Ali, was arrested on suspicion of offences under the Terrorism Act and possession of offensive weapons close to Downing Street on Thursday afternoon.
Police recovered several knives during the “intelligence-led” swoop, which took place yards from the scene of last month’s deadly attack by Khalid Masood on Westminster Bridge.
Believed to be a British national born overseas who went to school in Tottenham, north London, Ali reportedly joined a humanitarian mission to Gaza in 2010.
Following his dramatic arrest in front of crowds of tourists, he was taken for questioning at a south London police station where he remains.
A further six people, including a 16-year-old boy, are in custody after they were detained in an unconnected operation that saw a team of specialist firearms officers storm an address in Willesden, north London.
They were all arrested on suspicion of the commission, preparation and instigation of terrorist acts and taken to a south London police station for questioning.
One has been named in reports as Mohamed Amoudi, 21, a Yemeni-born British citizen who studied physics at Queen Mary University in east London.
A woman in her 20s who was shot during the raid is being held under police guard in hospital, where her condition was described as serious but stable. She is expected to be arrested when her condition improves.
Elite armed officers carried out a “specialist entry” into a terraced property in Harlesden Road shortly before 7pm on Thursday night.
Police fired CS gas into the address, which had been under observation as part of a current counter-terrorism operation.
Giving an update on the two operations, Metropolitan Police Deputy Assistant Commissioner Neil Basu said on Friday: “Due to these arrests that have been made yesterday, in both cases I believe that we have contained the threats that they posed.”
Mr Basu said an armed entry was necessary in Willesden “due to the nature of the intelligence that we were dealing with”.
The Independent Police Complaints Commission has been informed of the incident, as is routine for police shootings.
Home Secretary Amber Rudd praised police and security services, adding that the public’s role is “crucial in identifying suspicious activity” and urged anyone with concerns to contact authorities.
The official threat level for international terrorism currently stands at severe, indicating that an attack is “highly likely”.
It was revealed last month that security services have thwarted 13 potential attacks in less than four years.