Detectives in Croydon are appealing for information and witnesses after two women were robbed at knife-point of their gold necklaces in Thornton Heath.
During the incident the pillion passengers got off the bikes and are understood to have threatened the victims with a knife, before snatching a total four gold chains from the victim's necks.
A male witness, aged in his 70s, attempted to intervene and was thrown to the ground by one of the suspects.
The suspects then made off on the mopeds towards White Horse Road.
London Ambulance Service attended the scene. The witness who intervened was taken to south London hospital with a minor head injury where he was later discharged.
There have been no arrests and enquiries continue.
The suspects are described as three white male and one black male. All four wore motorcycle helmets.
Any witnesses or anyone with any information is asked to call Croydon CID on 101
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