There was an increased police presence outside the terminal after the MTA was made aware of plans for a pro-Palestinian protest at 5:30 p.m.
Protesters were seen gathering at Pershing Square on 42nd Street across from Grand Central.
Eventually, protesters began marching south past 40th Street before settling at Penn Plaza right next to Madison Square Garden.
Police say multiple arrests were made, but was not immediately clear how many.
The terminal has been the scene of disruptive pro-Palestinian protests in the past, so only commuters are being let inside the terminal.
The MTA told people to leave extra time to catch their trains.
It also advised commuters to check for scheduling updates on their phones.
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