A car swerved into passers-by then slammed into a barrier protecting Britain's Houses of Parliament in a suspected terror attack on Tuesday.
A 29-year-old British man, not previously known to the security services, was arrested at the scene by armed officers. Three people were injured.
Police, who initially said the man was refusing to co-operate, were holding him at a London police station on suspicion of preparing a terrorist attack.
Prime Minister Theresa May said the suspected attack was "shocking" and an "appalling incident" as she condemned the "twisted aim of the extremists".
