UPDATE (12:50 p.m. ET): One woman is reportedly dead in the attack, and the BBC says the assailant who stabbed the police officer is also dead.
UPDATE (11:35 a.m. ET): New reports say that police shot an assailant who'd stabbed an officer just after a car mowed down at least four people on Westminster Bridge.
There are reports of gunfire outside of Britain's Parliament in London.
Two people have reportedly been shot outside the building, and another 12 injured on nearby Westminster Bridge. Parliament was in session at the time.
Police are on the scene, but there's no word on how many gunmen there are or what their motive is. (USA Today)