A man has launched a sledgehammer attack on the security gates at the entrance of Downing Street forcing Whitehall to be closed off on Friday.
The man's car was investigated by the police's bomb disposal robot
A man was arrested by police officers at the scene at 0620 BST and taken to a central London police station.
The road from Parliament Square up to the Horse Guards' Building was sealed off and searched as a precaution but re-opened at 0730BST.
Police do not believe the incident is terrorist-related.
"This is not being treated as a terrorist incident. A more likely explanation is a small protest or mental illness," a Scotland Yard spokesman said.
The man's car, parked on a single yellow line right outside Downing Street, was investigated by the police's bomb disposal robot, according to staff at BBC London's Travel Unit who saw the vehicle on their cameras.
The Volvo 340 was parked on Whitehall, parallel to the Cenotaph, they said.
Downing Street has been the target for protests in the recent past.
In February this year a group of demonstrators were arrested for throwing white paint at the gates of Downing Street.
The group was concerned about the narrow remit of an inquiry to examine UK intelligence on Iraq's WMDs.
Five people were arrested for criminal damage.
Police are continuing to question the man over Friday's sledgehammer attack.