At 2:40 PM, a rented Hyundai Tucson mounted the pavement on the south side of Westminster Bridge and accelerated northward toward Parliament. The car struck pedestrians along 230 metres of the bridge, injuring more than fifty people. Andreea Cristea was knocked from the bridge into the Thames. The car then crashed into the railings of the Palace of Westminster's forecourt. The driver, Khalid Masood, a fifty-two-year-old British man, left the car armed with two knives. He ran through the open Carriage Gates and fatally stabbed PC Keith Palmer, an unarmed member of the Parliamentary and Diplomatic Protection Command, before being shot dead by a close protection officer.

The victims came from across the world. Kurt Cochran, the American tourist, died on the bridge; his wife Melissa survived with serious injuries. Aysha Frade, a forty-three-year-old teaching assistant born in London to a Spanish-Portuguese family, was killed as she walked toward her children's school. Leslie Rhodes, a retired window cleaner from Clapham, died in hospital after his life support was withdrawn. Andreea Cristea, who had fallen into the river, died on 6 April after her life support was also withdrawn. Her boyfriend survived his injuries. Among the more than fifty wounded were visitors from France, Romania, South Korea, China, Ireland, Greece, and the United States -- a reflection of Westminster Bridge's constant stream of tourists and Londoners going about ordinary life.

Keith Palmer was forty-eight years old. He had served fifteen years in the Metropolitan Police and had previously served in the Royal Artillery. He was an unarmed officer guarding the Carriage Gates, the entrance to the Palace of Westminster used by Members of Parliament. Palmer confronted the attacker and was fatally stabbed. He was posthumously awarded the George Medal and the Police Medal for Gallantry. A memorial to Palmer now stands in the grounds of the Palace of Westminster, near the spot where he fell. His funeral at Southwark Cathedral drew thousands of mourners, and flags were flown at half-mast across London.

Parliament was locked down for several hours after the attack, with MPs and staff confined inside the building. The acting Metropolitan Police Commissioner described it as a terrorist incident within minutes. Masood, who had a criminal record but no previous terrorism convictions, had been briefly investigated by MI5 years earlier but was deemed a peripheral figure. The investigation found he had acted alone, inspired by Islamist ideology but not directed by any group. In the days that followed, Londoners gathered on Westminster Bridge for a vigil, holding hands in a line across the span where the attack had taken place. A permanent memorial to the victims was later installed on the bridge. The attack prompted renewed debate about parliamentary security, but also demonstrated the courage of ordinary people and emergency responders who rushed to help the wounded in those first chaotic minutes.