The BBC is reporting that a police officer has been stabbed in the Houses of Parliament in central London.
The report says one woman has died while several others are critically injured after a car hit people on Westminster Bridge.
The alleged assailant was shot by armed police.
Footage on social media shows people lying in the road.
Metropolitan Police say they are treating it as a terrorist incident and buildings are in lockdown.
Mayor of London, Sadiq Khan, has issued a statement, saying “There has been a serious incident near to Parliament Square this afternoon which is being treated as a terrorist attack until the police know otherwise.
“I have spoken to the Acting Commissioner. The Metropolitan Police Service is dealing with the incident and an urgent investigation is underway. My thoughts are with those affected and their families.
“I would like to express my thanks to the police and emergency services who work so hard to keep us safe and show tremendous bravery in exceptionally difficult circumstances.
BBC
