A 15-year-old boy riding a train though Brooklyn was beaten and robbed of his cellphone, shoes and even his pants during a shocking gang attack caught on video.

Up to five teens pounced on the 15-year-old victim after he got onto the Canarsie-bound L train at the Broadway Junction station in East New York about 3:30 p.m. Friday, cops said.

The teen tried to flee the train at the next stop — the Atlantic Ave./Snediker Ave. station — but his attackers grabbed him on the platform, taking his shoes, pants and cellphone, police said.

A startling cellphone video taken during the brawl shows the teens beating their victim, then dragging the pantless teen back onto the train, where stunned straphangers called 911.

The lanky teen was sprawled out in his underwear on the subway car floor as concerned commuters tried to console him and give him water, the video shows.

Paramedics took the victim to Brookdale University Hospital with injuries to his head and body, officials said.

Cops were looking for the assailants Saturday.

Brooklyn community advocate Tony Herbert said the attack occurred when two gangs clashed.

“What is happening nowadays is a group of teens will walk up to another group of teens, whether they be two or more, and ask what gang they reppin, then in a flash of a second the assault begins,” Herbert said.