A gang of bikers terrorized a dad driving with his wife and baby daughter on the West Side Highway — chasing after their SUV and then dragging the man out and beating him to a pulp in front of his horrified family, authorities said.

One of the thugs even posted a video of the sickening beatdown on YouTube (images at left) after filming the pursuit using a helmet-mounted camera Sunday.

The attack seemed to come out of nowhere, authorities said. Alexian Lien, 33, of Manhattan, was driving the family’s Range Rover north near West 125th Street in Harlem when the gang, known as Hollywood Stuntz, surrounded the SUV at around 1:30 p.m., according to officials.

A biker pulled in front of the car and slowed down, forcing Lien to stop — at which point, as seen on the gang’s video, the bikers stop and a confrontation ensues.

“They take their helmets and they start to dent his car, and apparently his tires are slashed with a knife,” Police Commissioner Ray Kelly said at a press conference Monday.

The panicked dad gunned the engine, striking several bikers — one of whom was seriously injured — as he sped off, with the pack of thugs in hot pursuit, police sources said.

“He feared for his life and the lives of his wife and daughter,” a police source said.

The thugs chased him more than 50 blocks north, as seen on the 6-plus-minute video, which quickly went viral.

Lien was eventually forced to exit at West 178th Street in Washington Heights because his tires were so badly damaged, police said.

Off the highway, “he is stopped again by a group of motorcyclists, he’s taken out of the car. He is assaulted,” Kelly said.

One biker can be seen on the video ripping off his helmet and using it to bash in Lien’s driver’s-side window.

The crew pummeled Lien on the pavement in front of his wife, Rosalyn Ng, and their 2-year-old daughter, police sources said.

Lien, who also was slashed during the melee, was rushed to Columbia University Medical Center. He needed stitches to his face and chest and had two black eyes.

Sources said the injured biker was being prepped for surgery Monday night, and doctors feared he might be left a paraplegic.

Kelly said the gang of riders, known for their rallies and daredevil stunts, planned to congregate in Times Square that day. But he said the gathering was nixed by the NYPD because of past problems.

Last year, “they drove down Broadway [with] over 1,000 motorcycles, dirt bikes, quads,” Kelly said. “So there was an intention to do that again this year, but because of [police] checkpoints and enforcement in the outer boroughs, they weren’t able to make it.”

In shutting down the Times Square gathering, cops arrested 15 people, confiscated 55 motorcycles and issued 68 summonses to the bikers. None of those arrests were connected to the attack on Lien.

Police are still investigating the assault, including checking video from a nearby gas station where the bikers gathered afterward.

One of the rider’s jacket patches indicate he’s a member of a New Rochelle motorcycle club.