Wednesday afternoon, London Metropolitan Police reported being called to a "firearms incident" at Westminster Bridge. The leader of the House of Commons, David Lidington, reportedly told MPs, "It seems that a police officer has been stabbed; that the alleged assailant was shot by armed police. An air ambulance is attending the scene to remove the casualties. There are also reports of further violent incidents in the vicinity."

The Guardian's Vikram Dodd reported that the "early police view" is that there were two sites of attack — Westminster Bridge, where people were reportedly run over, and Parliament itself, where the policeman was reportedly stabbed and the attacker was reportedly shot.

Witness Richard Tice told Sky News that he "must have counted between eight and ten prostrate figures on the ground." Tice told Sky News he believed that an attacker used a motor vehicle: "a car or vehicle had driven along the whole pavement, knocking people over, and that's why there were many injuries, or many people laying on the ground." This account has not been confirmed.

Prime Minister Theresa May, who was apparently nearby when the incident occurred, was "rushed to a car," according to The Guardian. After the incident was reported, the House of Parliament suspended its operations, and MPs were asked to remain in the building. CNN reported that the Westminster tube station is temporarily closed because of the incident; in an aerial video from Sky News, traffic on the bridge, which includes several buses, appears to be at a standstill.

While much is still unknown about the incident, there are a number of tweets capturing the scene on the ground in London.