A £10,000 reward is on offer after the lives of passengers and staff were "put at risk" when a tram derailed in Mitcham.
No one was injured and staff were able to safely remove passengers from the tram.
Specialist officers attended and following further enquiries, it is believed an object had been placed on the track, causing the tram to derail.
Officers are appealing for anyone with information about this incident, or anyone who witnessed anything suspicious in the area at the time, to come forward as soon as possible to assist with their investigation.
Tram Operations Limited, who operate Tramlink, and Transport for London are offering a £10,000 reward for anyone who provides information which identifies an offender, and then leads to their successful conviction.
Detective Inspector Krishan Appannah from British Transport Police said: “Our investigation is still at a very early stage and we are currently looking to establish the full circumstances behind this object being placed on the track.
“Thankfully no one was injured however this is an incredibly concerning incident which clearly put the lives of passengers and staff at risk.
“I would urge anyone with information to contact us by texting 61016 or calling 0800 40 50 40 quoting reference 499 of 29/08/19. Alternatively call Crimestoppers anonymously on 0800 555 111.”
Mark Davis General Manager of London Trams, added: “Thankfully no customers or staff were injured in this incident, although we recognise it would have been very frightening for those involved. Safety on our network is paramount which is why we are working closely with the British Transport Police and Tram Operations Limited to help find the perpetrators.
"If anyone has information about this incident, please do contact the British Transport Police.”