U.K. Parliament Attacker Is Shot; Panic on Westminster Bridge

LONDON — Britain’s Parliament was placed on lockdown on Wednesday, plunging the country’s seat of power into turmoil, after an assailant stabbed a police officer outside the building before he was shot, and a motorist on an adjacent bridge plowed over at least five pedestrians, killing at least one.

The Metropolitan Police described the attack as a “a terrorist incident,” but many details, as well as the sequence of events, remained unclear. Security officers shot the assailant o...