An eight-year-old girl was killed and several other children were injured on Thursday when a car plowed into a primary school building in southwest London, triggering a major response by emergency services.

The crash at the private Study Prep girls' school in Wimbledon, was not being treated by police as terror-related, and the driver -- a woman in her 40s who stopped at the scene -- was arrested.

She was detained on suspicion of causing death by dangerous driving and was in custody, London's Metropolitan Police said.

"Very sadly, one of the children -- an eight-year-old girl -- died at the scene," said Detective Chief Superintendent Clair Kelland, her voice breaking with emotion.

"This is a very difficult time for everyone here at the school and the wider community," she told reporters at the site.

John Martin, chief paramedic at the London Ambulance Service, said they declared a major incident and sent 15 ambulances, specialist critical care paramedics and the London Air Ambulance.

Sixteen patients were treated on the spot and 10 taken to hospital. There was no immediate word on their ages or condition.

But the local member of parliament for Wimbledon, Stephen Hammond said he understood several casualties were "being treated as critical".

- 'Absolutely devastating' -

The crash was "extraordinarily distressing and tragic", he told the BBC.

The school, where fees are more than £5,000 ($6,300) a term, said it was too early to make any comment.

Aerial footage of the building -- not far from where the Wimbledon tennis tournament was taking place -- showed a Land Rover car stopped at an angle against the wall of the modern school building.

The gold-coloured vehicle was in a grassed area near what appeared to be coloured play mats and a table.

The police, ambulance and fire service were all called to the scene on Camp Road, near Wimbledon Common, after the incident just before 10 am (0900 GMT).

Witnesses and reporters said the road outside the school was a narrow one on which it would normally have been difficult to build up any speed.

The Study Prep school takes girls from aged four to 11. It is split into several sites, with the youngest pupils taught on Camp Road, near the Royal Wimbledon Golf Club.

Thursday was the last day of term for children aged four to eight, according to the school's website.

Health minister Steve Barclay called the incident "distressing". "My thoughts are with those sadly injured and everyone who has been affected," he added.

London Mayor Sadiq Khan called it "absolutely devastating".