LONDON — Police flooded central London streets and cordoned off the city’s government district Tuesday, after a speeding car plowed into cyclists and crashed outside Parliament in what authorities were treating as an act of terrorism.

With the driver in custody, counterterror detectives were working to uncover the motive behind what they suspect is the fourth vehicle attack in Britain — and the second on Parliament — in 18 months.

“Given that this appears to be a deliberate act, the method and this being an iconic site, we are treating it as a terrorist incident,” Assistant Commissioner Neil Basu, Britain’s top counterterrorism officer, told reporters.

A rooftop camera recorded the silver Ford Fiesta driving past Parliament and suddenly veering sharply to the left, striking cyclists waiting at a set of lights, then crossing the road and crashing into a barrier outside Parliament. Armed police surrounded the car within seconds, pulling a man from the vehicle. Police said the driver was alone and no weapons were found in the car.

Three people were hurt, none critically.

Police arrested the 29-year-old driver, a resident of England’s Midlands region who was not previously known to counterterrorism police or the intelligence services. Basu said the suspect was not cooperating with police.

Security Minister Ben Wallace said the man, whose name was not released, was a British citizen originally from another country.