"We are treating it as a terrorist incident until we know otherwise," Metropolitan Police tweeted.

According to preliminary data, an unidentified attacker mowed down several pedestrians on Westminster bridge and then crashed his car into a Westminster Palace rail.

Around the same time, a knife-wielding man attacked a police officer on duty outside the Parliament. Police opened fire on the attacker.

Sky News reported, citing its source in law enforcement agencies, that the attacker was the same in both incidents, according to preliminary data.

Reuters photo correspondent reported from the scene on a "dozen" victims.