South Woodford station in east London is under lockdown and nearby roads have also been cordoned off.
Witnesses said at least four police were at the scene
Officers have closed George Lane, outside South Woodford tube station.
The police cordon was set up at around 7am.
A Scotland Yard spokesman said: “Police were called at 02:43hrs on Saturday, 9 June, to reports of a shooting in George Lane, E18.
“Officers attended the location.
“Enquiries are ongoing to establish details of any person injured.
“Detectives from the East Area Command West are investigating alongside Trident colleagues.
“There has been no arrest at this early stage.”