LONDON — A knife-wielding man went on a deadly rampage in the heart of Britain’s seat of power Wednesday, plowing a car into pedestrians on London’s Westminster Bridge before stabbing a police officer to death inside the gates of Parliament. Four people were killed, including the assailant, and about 20 others were injured in what Prime Minister Theresa May condemned as a “sick and depraved terrorist attack.” Lawmakers, lords, staff and visitors were locked down after the man was shot by police within the perimeter of Parliament, just yards from entrances to the building itself and in the shadow of the iconic Big Ben clock tower. Just this weekend, hundreds of armed police took part in an exercise simulating a “marauding” terrorist attack on the River Thames. Wednesday was the anniversary of suicide bombings in the Brussels airport and subway that killed 32 people last year, and the latest events echoed recent vehicle attacks in Berlin and Nice, France. In the House of Commons, legislators were holding a series of votes on pensions when deputy Speaker Lindsay Hoyle announced that the sitting was being suspended and told lawmakers not to leave.