Two counter-terrorism operations in the space of a few hours. This is a fast-moving story - new facts are emerging all the time.
I'll keep this blog updated through the day with what we know, along with some analysis from the Home Affairs Unit, the BBC team that has covered terrorism for more than a decade.
- A 27-year-old man is still in custody on suspicion of preparing an act of terrorism and possession of offensive weapons - meaning knives.
- He was arrested at the bottom of Whitehall after being stopped and searched. Two addresses are being searched.
- He's being held under Terrorism Act powers - so police have 14 days to decide whether to charge or release him.
- Last night, armed officers deployed CS gas and forced entry to a home in Willesden, north-west London. This operation was unrelated to Whitehall.
- A woman who is 21 was shot and is in a serious but stable condition in hospital. Due to her injuries, she is not under arrest.
Six people have been arrested in connection with this second investigation - five at or near the property and a sixth person in Kent.
Again, all of them are being held under Terrorism Act powers so police have 14 days to charge or release.
Whitehall: Contained and controlled
It's understood that the man arrested in Whitehall yesterday is Khalid Mohamed Omar Ali.
He is 27 and is said to have gone to school in Tottenham in north London. He is a British national but its believed he was not born in the UK. He was known to the police and the security service MI5.
Its believed that it was a tip off from a concerned member of his family which led police to move in on him and arrest him yesterday. He was in possession of a number of knives when he was stopped close to Parliament Square.
Nobody else is under arrest in relation to this investigation. It doesn't mean police are not looking at other characters - but it looks like they are investigating what they think amounts to a lone actor (also known as a "lone wolf").
The manner of the arrest, and subsequent cordon in Westminster, indicates that they did not think there was a bigger going-on beyond the individual under investigation. Uniformed specialist firearms officers carried out a stop-and-search rather than a much more aggressive containment strategy.
Detectives will work through what they seized from the man, and anything they get from the addresses being searched - particularly from mobile phones, computers, or social media.
In 2010, the 27-year-old man travelled to Gaza as part of an aid convoy. The convoy travelled there by sea but encountered difficulties when 10 activists, including Mr Omar Ali, said that a ship captain held them captive and diverted them to Greece in disputed circumstances.
Willesden: Bigger public operation
The Willesden operation does seem to be the more important of the two.
It led to six arrests - three men and three women - following the deployment of the most specialist counter-terrorism firearms officers. You can see them in the pictures in the grey tops and balaclavas masking their identities.
The property had been "under observation" in the words of Scotland Yard. Every week there are observation operations that we never hear about - be it through electronic means or surveillance officers at suitable locations.
What's different about this one is the intelligence convinced Scotland Yard they needed to go in fast and hard to carry out arrests.
That means they feared there was some kind of threat that needed to be contained - a threat that required a large team of the most highly-trained available officers.
They fired CS gas canisters into the building, smashing holes in the top front window.
The 21-year-old woman who was shot is not one of those under arrest - but has been described by police as one of the "subjects" - security services speak for someone at the heart of an operation.
We don't know anything about how and why she was injured and that will be subject of a police watchdog investigation, as is always the case.
We also don't know if there was any police body-worn video from the scene.
How does this fit into the bigger picture?
The official threat level from international terrorism in the UK is "severe", which means that security chiefs think that attacks are highly likely.
That is obviously a truism given events in Westminster five weeks ago. Despite these two separate operations in the last 24 hours, officials have not ratcheted up the threat level to the highest status of "critical", meaning an attack is imminent.
That means the police and MI5 are confident that their current intelligence of known threats - and "known" is the operative word here - is good.
In his morning statement, Deputy Assistant Commissioner Neil Basu of the Metropolitan Police said his officers believe they have "contained the threats".
In other words, it is looking like coincidence that these two separate operations led to arrests on the same day.