A group of teen girls pummeled a female straphanger in Brooklyn when she shook her head disapprovingly as they harassed an elderly woman, authorities said.

The six girls were on board the L train near the Sutter Avenue station when the 28-year-old victim saw them arguing with the older woman around 3:30 p.m. Dec. 23.

But the teens quickly turned their attention to her when they saw her shake her head to express displeasure with their behavior.

The girls started punching and slapping the woman in the face and body before running off the train at Sutter Avenue, where surveillance cameras captured them making their getaway.

One of the punks stole the woman’s Samsung smartphone during the melee, police said.

The victim eventually got off the train and called 911.