An air ambulance landed in east London after a person was reported on the tracks at a Tube station.
A section of the Central line between Leytonstone and Newbury Park was shut as emergency services rushed to the scene in Wanstead.
The person was rushed to hospital with injuries the London Ambulance Service has described as "serious".
A large police presence, fire crews and an air ambulance were spotted near the station on Saturday morning.
An air ambulance was seen circling the area before landing on the nearby Wanstead green following the incident at about 11.30am.
There was no service between Leytonstone and Newbury Park eastbound following the incident.
No service from Leytonstone to Newbury Park - anticlockwise- person on the track.— Central line (@centralline) 8 April 2017
Something else has happened at Wanstead Station, chopper, three fire engines and five or six police vehicles. All started at 11.30am pic.twitter.com/GAZX522Gvp— Phil Mottram (@Motters19) April 8, 2017
One witness wrote on Twitter: "Something else has happened at Wanstead Station, chopper, three fire engines and five or six police vehicles. All started at 11.30am."
And another, Tom Knock, wrote: "The Air Ambulance landing on Wanstead green. My tube just hit somebody - awful."
An LAS spokesman said: "We were called just after 11.15am to reports of an incident at Wanstead station. We sent a number of staff to the scene including an an air ambulance.
"The patient was treated and stabilised at the station before being taken to a major trauma centre.
"They were in a serious condition."