Extra police officers are on patrol at South Yorkshire's railway stations today after a man was arrested on suspicion of terror offences following a crash in London.
Following the incident outside the Houses of Parliament earlier this morning, where a car collided with cyclists and pedestrians, extra British Transport Police officers have been drafted in to work at railway stations.
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Superintendent Chris Horton from British Transport Police, said: "We know incidents such as this are likely to cause concern, so our officers will be highly visible both on board trains and at stations.
"We are there to reassure the travelling public so please don’t be alarmed if you see our officers, including firearms officers, on your journey."
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A man in his late 20s is being held in custody at a south London police station following the collision just before 7.40am today.
Armed officers swarmed the scene following the suspected terror attack.
A silver Ford Fiesta swerved left, crossed oncoming traffic and a pavement before entering a small road and crashing into a security barrier outside the Houses of Parliament.
Scotland Yard is treating the collision as a 'terrorist incident'.
Images posted to social media showed a man, wearing a black puffer jacket, surrounded by police and being led away in handcuffs from the car.
There was nobody else in the vehicle and no weapons were found, the Metropolitan Police said.
Two people were taken to hospital, while a third person with minor injuries was assessed at the scene, London Ambulance Service said.
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Prime Minister Theresa May said her thoughts were with those injured and thanked the emergency services for their 'immediate and courageous' response.
A meeting of the Government's emergency cobra committee will be held at 2pm.
Witness Ewalina Ochab said: "I think it looked intentional - the car drove at speed and towards the barriers.
"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 car appears to have been driven through cyclists before ploughing into the security barrier.
Scotland Yard said: "At 7.37am on Tuesday August 14, a silver Ford Fiesta collided with a number of cyclists and pedestrians, before crashing into barriers outside the Houses of Parliament.
"The driver of the car, a man in his late 20s, was arrested at the scene by armed officers.
"He has been taken to a south London police station where he remains in police custody.
"He was arrested on suspicion of terrorist offences.
"There was nobody else in the vehicle, which remains at the scene and is being searched. No weapons have been recovered at this stage."
"At this stage, we are treating this as a terrorist incident and the Met's Counter Terrorism Command is now leading the investigation."
The Houses of Parliament are surrounded with security barriers of steel and concrete.
The terrorist threat against the UK is seen as unprecedented.
In addition to five attacks that occurred last year, authorities say they have stopped 13 Islamist and four extreme right-wing plots since the Westminster atrocity in March 2017.
Police and MI5 are running at least 500 live operations involving roughly 3,000 active 'subjects of interest' at any one time - while there is also a wider pool of more than 20,000 individuals who have previously featured in probes whose threat must be kept under review.
Earlier this year, the Government unveiled a refreshed counter-terror strategy.
Under the blueprint, MI5 intelligence will be shared with bodies outside the security community in an attempt to stop suspects before attack plots can crystallise, while anti-terror laws are to be strengthened to allow earlier interventions.