National Rail said Manchester Piccadilly station was unexpectedly shut from about 2pm today (Sunday).
Protesters had gathered on the tracks and were "attempting to climb overhead line equipment," it added in a post on Twitter.
The demonstration, which saw scores of flag-waving protesters make it onto the tracks, is thought to be in opposition to Turkey's war with Syrian Kurds.
A spokesman for British Transport Police said: "We were called to Manchester Piccadilly station shortly after 1pm on Sunday March 11 following reports of protesters trespassing on the tracks.
"The station has been closed to trains while officers at the scene work to ensure their safe removal and the reopening of the station."
The closure is causing delays across the region to rail services.
It is expected to continue until about 4.30pm today.
National Rail said services are accepting tickets along other "reasonable routes" for passengers affected at the station.
In a statement on its website, National Rail said: "A trespass incident at Manchester Piccadilly is causing major disruption to trains at the station.
"Trains may be amended, cancelled or delayed by up to 60 minutes.
"A normal service is expected to resume by 16:30."
Turkey launched a solo military offensive against the Syrian Kurdish People's Protection Units or YPG two months ago to clear them from Afrin in north-western Syria.
Turkish president Recep Tayyip Erdogan has urged Nato to come to Turkey's aid, saying its borders are "under threat right now".