The prime minister’s office confirmed Johnson was in the car and that there were no reports of injuries.
Video footage shows Johnson’s silver Jaguar pull onto Downing Street behind a police motorcycle leading the convoy when a man runs into the road.
Both the lead motorcycle and Johnson’s car hit the brakes suddenly, causing a Range Rover SUV carrying his security detail to ram into the back of the Jaguar. A dent could be seen on the trunk of the vehicle.
Images from the scene show the protester fall to the pavement before he is taken away by several police.
The man is believed to have been part of a pro-Kurdish group protesting Turkey's treatment of Kurdish rebels, Sky News reported.
The man was arrested on charges of disorderly behavior under Section 5 of the Public Order Act and for obstructing the highway, the Metropolitan Police said in a statement.
Johnson was leaving Parliament after participating in the weekly questions session. It's been about six weeks since Johnson has been back at work following his bout with COVID-19.
The Parliament gate near where the incident occurred was also the scene of a terror attack in 2017 that killed 5 people, including a police officer, and injured 50 others.
Fox News’ Greg Palkot contributed to this report.