Boris Johnson’s ministerial car was involved in a shunt after a protester blocked the road outside the Palace of Westminster.
The demonstrator, who had been protesting about Turkish action against Kurdish rebels, ran into the road outside the gates to Parliament as Mr Johnson left following Prime Minister’s Questions.
This forced the lead vehicles in the convoy – a police motorbike and the Prime Minister’s Jaguar – to stop, with a support vehicle running into the back of the luxury saloon carrying Mr Johnson.
The Metropolitan Police said there were no reported injuries and a man had been arrested for offences under Section 5 of the Public Order Act and for obstructing the highway.
The vehicles drove off shortly afterwards in the direction of Downing Street, with a large dent visible on the rear of the Jaguar.
A Scotland Yard spokesman said: “Two police vehicles were involved in a collision in Parliament Square.
“The police vehicles were part of a security movement.
“A pedestrian is reported to have stepped onto the road, causing the vehicles to suddenly stop, which has led to two of the vehicles in the convoy being involved in a damage-only collision.
“No injuries have been reported.”