A car mowed down around eight to ten people "at speed" on Westminster Bridge moments before a suspected 'knife attack' occurred at the Houses of Parliament.
Details are still emerging of what's being described as a separate 'violent incident' in the vicinity of the attack - but while there is no official line yet, numerous commentators are linking them.
It's also being called a "mass casualty event" where a number of people have sustained "catastrophic injuries' according to one doctor treating them - who also said at least one woman has died, yet to be officially confirmed.
Police say they are treating this afternoon's Westminster attack as terrorist related "unless they discover otherwise".
Shocking video has emerged showing chaotic scenes and a number of people lying injured on the bridge.
One terrified eyewitness said that a vehicle had 'deliberately' mounted a pavement a hit people before crashing into a barrier.
They said they saw between eight and ten people lying in the road injured.
Another witness, who did not want to be named, said: "I saw a car go out of control and saw people lying on the floor, about 10 people maybe".
It's also being claimed that people dived off the bridge to avoid the oncoming car - yet to be confirmed, although pictures of people lying injured below the bridge have emerged.
There are unconfirmed reports of one person being spotted lying face down in the River Thames.
A photo has emerged of what looks like a body lying under a bus on the bridge - but there have been no confirmations of any fatalities.
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An eyewitness said a Hyundai 4x4 carried on down towards Big Ben and crashed into railings after colliding with a number of pedestrians.
The driver was seen sprinting away from the scene and gunfire - believed to be from police - was heard afterwards.
It is not known if the driver was their target - but a man of similar description was shot by police nearby shortly afterwards at 2.42pm after allegedly stabbing a police officer while trying to get into the Parliament building.
Witnesses reported hearing a "number of loud bangs" and Parliament was immediately closed down.
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Social media commentators say a number of bodies have been seen on the bridge - yet to be confirmed by the police.
The video shows a number of people lying on the floor being aided by passers by.
Emergency services are attending all the injured - and are taking people to St Thomas' to be treated.
People fled in terror as the dual incidents - including an alleged assailant shot by police outside the Houses of Parliament where a police officer was stabbed - unfolded.
Witness Rock Longley said: "A guy came past my right shoulder with a big knife and just started plunging it into the policeman."
The man witnesses are claiming was behind the attack is described as Asian and in his 40s - brandishing 'a knife about seven or eight inches long."
A 30-second clip posted on Twitter by @sikorskiradek showed a man lying in the bus lane, with several people running towards him.
It then pans to another man lying in the middle of the road holding his left leg while two people kneel over him.
A knocked-down orange cone can be seen at the edge of the pavement in the background.
@sikorskiradek posted: "A car on Westminster Bridge has just mowed down at least 5 people."
One white van and a motorbike were still in the road, with onlookers appearing to walk and run towards the injured.
A number of plain clothes detectives have flooded the scene alongside emergency services.
Parliament is in lockdown and there are multiple ambulance crews in attendance.
A line of red buses and several ambulances could be seen on Westminster Bridge.
Alex Wafer, 47, who was cycling nearby and saw the aftermath, said: "At the end of the embankment it appears as though there is an ambulance.
"It looks as if they are treating somebody.
"And just below, it looks like there may be another paramedic treating someone else."
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The Air Ambulance landed in Parliament Square - among dozens of ambulances which rushed to the scene.
We'll be bringing you the very latest updates, pictures and video on this breaking news story.