Many on Twitter are using the hashtag Pray for London to show their sympathy for victims, after the attacks in Westminster this afternoon.
A car has mowed down several pedestrians on Westminster Bridge and an attacker wielded a knife and attacked a policeman before being shot.
Police said a "full counter-terrorism inquiry" was under way and there have been "a number of casualties, including police officers."
One woman has been confirmed dead after the attack on Westminster Bridge, and several other people are reportedly seriously injured.
View the reaction on Twitter below