A man is being quizzed by police after a car ploughed into the gates of Downing Street, slightly injuring a mother and her young child.

The red Vauxhall car collided with the security gates facing on to Whitehall at about 1230GMT but did not break through, Scotland Yard said.

The Prime Minister and his family were not in Number 10 at the time. Chancellor Gordon Brown was also not at home.

Officers are investigating whether the driver deliberately attempted to smash down the heavy security barrier which protects the Prime Minister's residence, or had simply lost control of the vehicle.

A Yard spokeswoman said: "The vehicle was in collision with the gates to Downing Street. A woman was taken to hospital as a result of the collision suffering from leg injuries and the driver is being questioned."

'Saved children'

Eye-witness Sertac Bahri, who was in a Turkish protest opposite Downing Street, said the injured woman was with two young children.

"We heard the car skidding in and jumped out to look.

"I saw the woman push one of her children out of the way to save them from getting hit, but the car went into the woman instead. She flew up in the air legs first and landed about five metres away.

"The police rushed in from all directions and opened the car door and pulled out a man who they pushed to the floor and handcuffed."

Mr Bahri said the man looked to be between 30 and 35 years of age and had blond spiky hair.

Peter Saacs, 55, who was driving a Stagecoach bus past Downing Street at the time, said: "The driver (of the car) pulled up as if to show someone Downing Street.

"Then he was wheel-spinning in front of me and pulled out and went round the back of our bus and into the gates."

The car, which suffered damage to the front right side, was taken away by police shortly afterwards.

A support strut on the gates was broken in the crash and the bolt connecting the gates to the ground was bent.

Car skid marks were also clearly visible, although police had cordoned off the immediate area.

'No breach of security'

A Downing Street spokeswoman said: "The Prime Minister and his wife were not at home at the time of the incident. The Prime Minister has been informed and he has asked to be kept informed of the progress of the injured woman."

Scotland Yard said the car driver was being held on suspicion of causing criminal damage.

A spokesman added: "There is no question of a breach of security at Downing Street."