Transport for London (TfL) said there was no service on the line between King’s Cross St Pancras and Seven Sisters while emergency services dealt with the incident.
A post on TfL’s website said: “No service between King's Cross St. Pancras and Seven Sisters while emergency services deal with a casualty on the track.”.
In an update, TfL said that tickets were being accepted by London Buses as well as on the Weaver Line and the Suffragette Line.
Great Northern, Thameslink, Greater Anglia, South Western Railway and Southeastern services were also accepting tickets.
A “casualty on the track” has brought trains on the Victoria Line to a standstill.