A man is in a critical condition after he was attacked by a group of males who were trying to steal his moped.
The Met said the 46-year-old approached them as they were attempting to take it from a car park near shops off Battersea Park Road, south-west London.
Paramedics attended and he was taken to hospital with a serious head injury just after 16:30 BST on Saturday.
The moped was not stolen and is being forensically tested, after what police have described as a “cowardly attack”.
A 16-year-old was arrested on suspicion of causing grievous bodily harm and has been released on police bail to a date in early June, pending further inquiries.
The victim’s next of kin have been told.
The group were hanging around the area on bicycles shortly before the incident.
Investigating officers want to hear from anyone who was in the area – there is a busy Tesco store and other shops nearby – and anyone with dashcam footage is urged to come forward.
The senior investigating officer, Det Ch Insp Mark Cranwell, said: “I want to reassure the community that there is an urgent and extremely thorough police investigation under way, and I am determined to find those responsible.”
He added: “I am certain that several people – probably local to the area – will know what happened and who was responsible for this attack on the victim and on their own community. I need them to come forward and share information with me.”