A man was rushed to hospital after being hit over the head during a robbery outside a petrol station.
London Ambulance Service sent one crew to 28 Bishopsford Road, Morden, to reports of an assault just before 10am.
Police attended the Murco garage shortly after, but failed to catch the criminal.
Paramedics treated a man in his 40s for a head injury and he was taken to hospital.
His injuries are not believed to be life-threatening.
No-one was arrested but detectives are carrying out further investigations.