Cyclists and pedestrians have been injured after a car crashed into the security barriers outside Parliament.
The vehicle is said to have sped into the barriers, at an estimated speed of 40 miles per hour, in what is being considered to be a deliberate attack.
"We have treated two people at the scene for injuries that are not believed to be serious and have taken them to hospital", the London Ambulance service said in a statement.
It is believed that the two people hospitalised were cyclists.
The car collided with the barriers outside the Houses of Parliament at 7:37am.
Scotland Yard is said to be treating the incident with an "open mind", but the Metropolitan Police's counter-terrorism command is leading the investigation.
A male driver of the car - a silver Ford Focus - was arrested on suspicion of terrorist offences at the scene, by armed officers who surrounded the car - which was crumpled in the crash.
The man, who is in his late 20s, has been taken to a south London police station.
There was nobody else in the vehicle, which remains at the scene and is being searched. No weapons have been recovered at this stage.
The streets around Parliament Square, Millbank and Victoria Tower Gardens have been cordoned off Westminster Tube station is closed.
A selection of police vehicles and ambulances are attending the scene.
The Metropolitan Police have asked for anyone with information to call 0800 789 321, and anyone with footage of upload them at www.ukpoliceimageappeal.co.uk.
Extra police officers - both armed and unarmed - have been deployed to the streets around central London and police say members of the public should remain vigilant and call 999 if they see anything suspicious.
Cycling Weekly's Tech Editor Michelle Arthurs-Brennan is a traditional journalist by trade, having begun her career working for a local newspaper before spending a few years at Evans Cycles, then combining the two with a career in cycling journalism.
When not typing or testing, Michelle is a road racer who also enjoys track riding and the occasional time trial, though dabbles in off-road riding too (either on a mountain bike, or a 'gravel bike'). She is passionate about supporting grassroots women's racing and founded the women's road race team 1904rt.
Favourite bikes include a custom carbon Werking road bike as well as the Specialized Tarmac SL6.