Four people are dead and more than 20 have been injured in the attack at Westminster, Scotland Yard's top anti-terror officer Mark Rowley has confirmed.
Those killed include the policeman at the scene and the suspected attacker, who was killed by police firearms officer.
Mr Rowley told reporters outside New Scotland Yard, just yards from the scene of the attack: "This was a day we've planned for but hoped would never happen. Sadly it is now a reality."
Dr Charles was in a building overlooking the area where the attack happened.
he reports hearing a loud bang, and seeing a Hyundai car crashed into the railings. It is thought the car is registered in Chelmsford.
he says the driver ran down the pavement brandishing a knife. Listen to his eyewitness report below.
North Norfolk MP Norman Lamb was caught up in the events at Westminster as they unfolded.
Two people are confirmed to have died after a knifeman brought terror to the heart of Westminster before being shot by armed officers.
Mr Lamb has spoken to ITV News Anglia, as he was on his way back to Portcullis House when the attack took place.
It is incredibly distressing. The thought that an innocent civilian has lost their life on Westminster bridge and others are catastrophically injured is just awful.
It is just really shocking. There was a sense and a certain inevitability that something like this was going to happen at some stage.
The first instinct is that your thoughts go out to loved ones and those that have lost their lives.
I desperately hope that this isn't a shape of things to come, but obviously we wait to hear about the security assessment and what has happened.
This is the first photo of the man who is believed to have been the attacker in a terrorist incident in Westminster.
The man is shown on a stretcher receiving medical treatment. In the image, weapons that are believed to be knives have been circled in red.
The car reportedly used as part of a terror attack in Westminster was registered in Chelmsford.Read the full story ›
Extra police officers will be placed on Greater Anglia and c2c trains this evening, following a terrorist attack in Westminster.
British Transport Police in Essex tweeted the announcement on their social media page, to offer reassurance to passengers travelling to and from London.
At least one person is known to have died from their injuries in the attack.
The London Ambulance Service has said that it has treated ten patients involved in the terror incident so far, and that multiple hospitals are on alert in the capital.
Bedfordshire and Essex police are reporting a higher than usual number of calls to their control rooms after the terror attack in London.
Bedfordshire police are asking people to follow the Metropolitan Police twitter feed for updates and Essex police have said to only call them in an emergency.
East Midlands Ambulance Trust have offered their thoughts to the 999 staff dealing with the incident.
Awful incidents here at Westminster. Thoughts very much with those hurt. Atmosphere calm within Chamber. https://t.co/4RYZOM6cTO
Thinking of those injured in London today. Grateful to security at Parliament for all they do to keep us safe.
A picture taken at the scene of a suspected terror incident near Parliament shows the moment an assailant is contained by armed police officers.
The suspected attacker crashed his car into pedestrians on Westminster Bridge before stabbing a police officer at the Houses of Parliament.
Eyewitness Frances, who was in the area when the incident happened, told ITV News reporter Emma Murphy that her partner was in parliament.
She said everyone in Parliament is still locked in, and said her partner had been in touch: "He understood that there'd been an incident on Westminster Bridge, and that a police officer had been stabbed.
"He texted to say that he's still in there and they won't let them out, and there's a suspected package in a car, so at the moment, I'm just going to wait for him to come out.
"I don't know how long he's going to be, but it's obviously very important for them to secure the area before they let them out."