Boris Johnson’s convoy was involved in a minor collision outside Parliament when a protester ran towards the cars.
This forced the lead car in the convoy to brake suddenly causing the escorting vehicle to collide with the saloon car carrying Mr Johnson
A large dent was seen in the prime minister’s Jaguar as the convoy drove off towards Downing Street.
No 10 confirmed Mr Johnson was in the car and said there are no reports of any injuries.
Asked later about the incident, Culture Secretary Oliver Dowden said Mr Johnson was “wholly unscathed”.
The demonstrator had been protesting about Turkey’s action against Kurdish rebels.
The police said a man was later arrested at the scene for offences under the Public Order Act and for obstructing the highway.
The prime minister was in Parliament on Wednesday, for the weekly Prime Minister’s Question session.