LONDON — A car struck and injured several pedestrians and cyclists, then crashed into a security barrier just outside the Houses of Parliament in London early Tuesday, in what the police said they were treating as a terrorist attack.

About a dozen armed police officers swarmed a silver Ford Fiesta and pointed their weapons at it, before handcuffing the driver, who was arrested on suspicion of terrorist offenses. The police said that the driver, in his late 20s, was the only person in the car, and that no weapons had yet been found in the vehicle.

Neil Basu, the counterterrorism chief of the Metropolitan Police, said the department was treating the episode as one of terrorism, “given that this appears to be a deliberate act,” the sensitive location and the method used. Parliament has been attacked before, and past terrorist attacks have involved cars driving into pedestrians.