A man has been arrested after a auto crashed into the security barriers outside the British Houses of Parliament.
"We're still trying to piece it together", a spokesman said.
The United Kingdom's Metropolitan Police confirmed a man had been detained about 7.30am local time.
Officers could be seen surrounding the vehicle before leading a man away in handcuffs. One of those attacks saw an Islamic extremist mow down pedestrians with a vehicle before dismounting and killing a police officer guarding the Palace with a knife.
Another witness, Ewalina Ochab, told the Press Association: "I think it looked intentional - the auto drove at speed and towards the barriers".
"I was walking on the other side [of the road]". No one was being allowed entry to parliament.
Westminster Underground station, close to parliament, was closed and the building was cordoned off. As parliament is in recess for summer, most lawmakers are not using the building. Ambulances and firefighters are also attending the scene. The cordon is stretching significantly.
A Scotland Yard statement reads: "At 0737 hours today, a auto was in collision with barriers outside the Houses of Parliament".
Masood rammed his vehicle into a crowd on nearby Westminster Bridge, killing four people.
There are "dozens" of armed police at the scene, Sky News reported.