A 22-month-old baby girl in a pushchair has died following a crash involving a car in northwest London.
Police were called shortly after 4pm on Sunday to reports of a collision involving a car and three pedestrians in Eastcote Road, Ruislip.
The child was pronounced dead at the scene.
Another pedestrian, a man in his 30s, was taken to hospital and is in a critical condition.
A man in his 40s has been arrested on suspicion of causing death by dangerous driving, causing serious injury while driving, and driving under the influence of drugs.
He is currently in custody.
Metropolitan Police officers - including some who were off duty - and paramedics were at the scene.
The child's family has been informed and they are being supported by specially trained officers.
A post-mortem examination is set to take place.
Police are now appealing for witnesses or anyone with dashcam or CCTV footage to come forward.
Detective Sergeant Sarah Donegan of the Met Police said: "Our thoughts are with the child's parents at what must be an unimaginably difficult time.
"We have launched an investigation to establish the circumstances of what happened and I am asking anyone who was in the vicinity at that time who may have seen something, or has dashcam footage or CCTV, to come forward and help us with our investigation."
Witnesses are urged to call 020 8246 9820, or 101 ref CAD 4892/25OCT20. You can also contact the independent charity Crimestoppers on 0800 555 111