"A number of pedestrians" have been injured after a car crashed into security barriers outside parliament.
A man has been arrested after the vehicle drove into security barriers, the Metropolitan Police confirmed.
The driver was detained at the scene by armed police, wearing a white shirt and a black hooded puffer jacket.
The black man was led backwards away from the silver Ford after police held him at the scene for a few moments.
In a statement, Scotland Yard said: "At 7.37am today, a car was in collision with barriers outside the Houses of Parliament.
"The male driver of the car was detained by officers at the scene. A number of pedestrians have been injured. Officers remain at the scene.
"We will issue further information when we have it."
:: Westminster crash: What we know so far
At 8.40am they added: "At this stage, officers do not believe that anybody is in a life-threatening condition. Cordons are in place to assist the investigation. Westminster tube station is closed."
Reports suggest people are being held inside Westminster station at police request.
1:13According to Vincent McAviney, the police radio reported the incident would be treated as "counter-terror".
Ewalina Ochab told the Press Association: "I think it looked intentional – the car drove at speed and towards the barriers."
She said: "I was walking on the other side [of the road]. I heard some noise and someone screamed. I turned around and I saw a silver car driving very fast close to the railings, maybe even on the pavement."
"The person driving did not go out" of the car she added.
According to witness reports, cyclists have been seen "lying in the road" after the crash.
Streets around Parliament Square, Millbank and Victoria Tower Gardens have been cordoned off, with the cordon moving further away from the scene.
Sky's political reporter Aubrey Allegretti said he has been told the area around Millbank has been "sanitised".
There are "dozens" of armed police and 10 cars on the scene, as well as two police dogs waiting nearby.
Armed police were seen leading the driver away in handcuffs, and a number of ambulances are at the scene.
Parliament is in summer recess, and so there are fewer politicians working in London, but there are numerous other people working nearby, including political journalists.
MI5's headquarters is also nearby.
One man who witnessed the crash told Sky News it was "very very dramatic".
He added: "I seen smoke coming out of a vehicle. It's all been high speed – you've got the specialists coming in… it's been dramatic."
Bus driver Victor told Sky News: "I stopped the bus because of the crash, and police told us to 'move back'.
"I think it's not a regular traffic accident, because there's a barrier there. So I don't know why someone would drive into the barrier.
"I saw someone was arrested, but I couldn't see him closely. He was taken away very quickly."
Sky's home affairs correspondent Mark White said: "This has taken an ominous turn, with the confirmation that pedestrians have been injured.
"Anyone who knows this area will know that there is a filter off that allows vehicles to go into parliament itself… there is a big barrier that comes across the road, designed to protect the buildings."
Tom Parmenter, Sky's political correspondent, said: "The police response is very significant indeed. In the initial stages we saw sniffer dogs.
"Police drafting in more resources as we speak.
"All the time we are getting officers arriving at the scene. Some of them plain clothes, some of them in uniform. The cordon is stretching significantly.
"The judgement of the emergency services is that their response is proportionate to the incident they are dealing with.
"We still don't know the motive for why that car drove at those barriers."
More from London
Steel and concrete barriers were added outside parliament after the 2017 Westminster Bridge terror attack.
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