The trespasser has now been removed from the railway by British Transport Police but National Rail say the incident has caused "major disruption" to morning services.
The disruption at the station is expected to continue until around 12pm, said the rail company. All routes into Manchester Piccadilly are affected, including Northern, Avanti West Coast and TransPennine Express services from Preston, Wigan and Blackpool North.
A spokesman for National Rail said: "The trespasser near Manchester Piccadilly has now been removed from the railway by the British Transport Police.
"Trains at the station may be delayed by up to 60 minutes or cancelled while services start to return to normal.
"Major disruption to trains at the station is expected to continue until approximately 12.30pm."
Alternative travel advice for Northern passengers
Northern say tickets for routes into and out of Manchester Piccadilly will be accepted on alternative Northern services to / from Manchester Victoria "via any reasonable route".
TransPennine Express services said it will also accept tickets to / from Manchester Victoria whilst disruption continues at Piccadilly station.
Check before you travel
You can check you the status of live services using the National Rail Enquiries real-time Journey Planner here.
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