An active terror plot was foiled when police launched an armed raid which saw a woman shot and injured, detectives believe.
Six people were arrested after a team of specialist firearms personnel stormed an address in Willesden, north London, on Thursday night.
A woman in her 20s was shot during the operation and remains under police guard after being taken to hospital in a serious but stable condition.
The activity came hours after officers arrested a man carrying a bag of knives in Whitehall, close to Downing Street, as part of a separate, unconnected investigation by Scotland Yard and MI5.
Giving an update on Friday, Deputy Assistant Commissioner Neil Basu, the senior national co-ordinator for counter-terrorism policing, said: "Due to these arrests that have been made yesterday, in both cases I believe that we have contained the threats that they posed."
Pressed by reporters on whether they had foiled an active plot, Mr Basu replied: "Yes."
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, Mr Basu said.
He added that an armed entry was necessary "due to the nature of the intelligence that we were dealing with".
The senior officer said: "During the course of that operation one of the subjects of that operation - a woman - was shot by police; she remains in hospital.
"Her condition is serious but stable. Because of her condition she has not yet been arrested. We are monitoring her condition closely."
Neighbours recalled hearing "at least six" shots and screaming as police entered the address.
A mother-of-one who lived next door to the raided house, who gave her name only as Alexandra, saw officers with "gas masks and snipers".
"We heard 'bang, bang, bang, bang', went to the window and just saw a number of armed police just there with their guns pointing at our next-door neighbour's window."
She said the woman was "screaming really loud" and described her neighbours as a "standard Muslim couple" of whom she "never suspected anything at all".
Alex Paton, 50, who also lives in Harlesden Road, said he saw a woman "getting all cuffed up and put in a white suit", and described seeing "all sorts of people running around - there was coppers and masked men with guns and there was a guy running down the road with a gas mask and a machine gun".
The Independent Police Complaints Commission has been informed of the incident, as is routine for police shootings.
A total of six people were arrested in connection with the raid, including five at or near the address in north London and one person at an address in Kent.
Two of the arrests were made when a man and a woman, both aged 28, returned to the address later that night, Mr Basu said. Searches are continuing at three London addresses.
It followed the arrest of a bearded man carrying a rucksack containing at least three knives just yards from the scene of last month's deadly attack by Khalid Masood in Westminster
Scotland Yard said the 27-year-old was arrested "following a stop and search as part of an ongoing operation".
It is understood the arrest was "intelligence-led" and made as part of a joint operation by MI5 and counter-terrorism police.
The circumstances of the swoop suggest the man was under some form of surveillance before his arrest.
The Daily Telegraph reported that it is understood the suspect's family had become concerned about his behaviour and reported him to the authorities several weeks ago.
Detectives are continuing to question the man, while two searches are being carried out at addresses in London.
Mr Basu said that, with last month's Westminster attack "so fresh in people's minds", he wanted to reassure the public that officers were working around the clock to identify would-be terrorists.
Thanking the public for their "continued help and support", he added: "No matter how hard we try, it is communities that will defeat terrorism and we rely on your vigilance."
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