An assailant armed with a knife has been shot outside the Houses of Parliament in London, and at least a dozen people have been injured on Westminster Bridge. The number of casualties is unclear. Stay up to date with the latest updates on the Westminster terror attack on our blog.

The leader of the House of Commons, David Lidington, said in the chamber that an assailant who stabbed a policeman had been shot by police. Amid confusing scenes, it appeared the incident may have unfolded in several locations, including the busy bridge where tourists often congregate to take pictures of Big Ben and other attractions. The incident took place on the first anniversary of attacks on Brussels in Belgium.

Houses of Parliament Westminster Bridge
Paramedics treat an injured person after an incident on Westminster Bridge in LondonToby Melville/Reuters
Houses of Parliament
Injured people are assisted after an incident on Westminster Bridge in LondonToby Melville/Reuters
Houses of Parliament
A woman assist an injured person after an incident on Westminster Bridge in LondonToby Melville/Reuters
Houses of Parliament
A member of the public is treated by emergency services near Westminster Bridge and the Houses of Parliament in London,Daniel Leal-Olivas/Getty Images
Houses of Parliament
A member of the public is treated by emergency services near Westminster Bridge and the Houses of Parliament in LondonDaniel Leal-Olivas/AFP
Houses of Parliament
People leave after being evacuated from the Houses of Parliament, LondonDaniel Leal-Olivas/Getty Images
Houses of Parliament
People leave after being evacuated from the Houses of Parliament in central LondonDaniel Leal-Olivas/AFP
Houses of Parliament
An air ambulance lands in Parliament Square during an incident on Westminster Bridge in LondonToby Melville/Reuters
Houses of Parliament
Armed police respond outside Parliament during an incident on Westminster Bridge in LondonToby Melville/Reuters
Houses of Parliament
Armed police respond outside Parliament during an incident on Westminster Bridge in LondonStefan Wermuth/Reuters
Houses of Parliament
A medic is seen near Westminster Bridge and the Houses of Parliament in LondonCarl Court/Getty Images
Houses of Parliament
An armed police officer runs accross the road during an incident on Westminster Bridge, LondonToby Melville/Reuters
Houses of Parliament
A man lies injured after a shottingt incident on Westminster Bridge in LondonToby Melville/Reuters
Houses of Parliament Westminster Bridge
A police officer stands guard near Westminster Bridge and the Houses of ParliamentJack Taylor/Getty Images
Westminster shooting
A police officer stops traffic as the Jagaur car of British Prime Minister Theresa May (R) is driven away from the Houses of Parliament, LondonDaniel Leal-Olivas/Getty Images
Houses of Parliament
Armed police officers guard at a police cordon outside the Houses of Parliament in LondonDaniel Leal-Olivas/AFP