Masked anti-cop protesters flooded Grand Central Terminal at the start of rush hour Friday in an attempt to shut down the commuter hub — but the whole thing soon fizzled out.

There were at least 12 arrests by MTA police following a skirmish between cops and demonstrators, who refused to remove their masks despite repeated orders from police.

Further arrests were expected as protesters headed out of Grand Central and into Bedford-Stuyvesant Brooklyn, police sources said.

Some of the agitators attempted to approach a nearby subway entrance, only to find it blocked off. Others chanted loudly while holding signs with messages like “Money for elevators not more cops,” “F–k the police fight the power” and “No fare no cops.”

Grand Central commuters were unimpressed with the 5 p.m. attempt to disrupt service, which dissipated within 30 minutes.

“I think it’s absurd, things aren’t free; things cost money,” said one Westchester-bound traveler. “Asking for free stuff, it’s like they’re 12 year olds. They’re acting like children!”

Friday afternoon’s demonstration was just the latest salvo in a full day of anti-cop, anti-fare protests organized by the group Decolonize This Place, whose goals include “no cops in the MTA, free transit [and] no harassment.”

Protesters across the city vandalized turnstiles, trains and stations with anti-cop messages.

Two women were also arrested, summonsed and released at around 11 a.m. after posting “F—k the police” signs inside Bronx Criminal Court, cops said.

As of 5 p.m., police were also looking for an unknown individual who used a metal chain to shatter the screen of one of the MTA’s new OMNY fare machines at West 50th Street and Eighth Avenue.

Each OMNY machine costs the MTA $2,000 to replace, sources said.

MTA Chief Safety Officer Pat Warren said the protests “divert valuable time, money and resources away from investments in transit services that get New Yorkers to their jobs, schools, doctors and other places they need to go.”

“This demonstration activity follows the dangerous pattern of previous activities that have resulted in vandalization and defacement of MTA property – clearly violating laws,” Warren said in a statement.