A Cambridge man who says he was blasted with pepper spray while trying to film a protester being arrested on a highway on-ramp Thursday night posted a video of his dustup with a trooper online, prompting state police to launch an internal investigation.

“The Massachusetts State Police are aware of a video posted online that appears to show a trooper using pepper spray on a protester in Boston,” state police spokesman David Procopio said last night. “We have begun an internal investigation into the action seen in the video to determine whether it constitutes a violation of departmental policy. The department’s investigation will be thorough and unbiased.”

Kin Moy, 24, told the Herald that he and some friends joined hundreds of others Thursday who were protesting the killings of unarmed black men by police in Ferguson, Mo., and Staten Island, N.Y. Around 9:30 p.m., they made their way onto the Interstate 93 on-ramp near South Station, he said.

“We saw a bunch of officers start dog-piling this guy who was not resisting arrest and it seemed to me to be a little rough,” Moy said. “I wanted to film all of it just in case, and as I was backing up and complying with his demands but still trying to film everything, he sprayed me.”

Moy posted the video, which he recorded on his cellphone, to YouTube. It had been viewed more than 2,500 times last night.

“I don’t think that all cops are bad. I just think that there are too many that are getting away with things that they ought not to be without facing repercussions and that’s what these protests were all about for me,” Moy said. “Ironically enough, I felt that the officer who pepper sprayed me was being an example of that.”

In the 39-second video, a state trooper confronts Moy, yelling “Go, now, you’re going to get sprayed.”

The trooper appears to hold a canister and spray Moy three times before the Cambridge man turns the camera on himself, showing an orange liquid across the side of his face.

State police posted a message on the YouTube video, asking Moy to contact them.

“We are in the process of trying to contact the person who posted the video, whom we also believe is the person who was sprayed,” Procopio confirmed.

Moy, a Mixed Martial Arts fighter, said he is seeking help from the American Civil Liberties Union.

“I do intend to reach out to the state police at some point,” Moy said. “I intend to meet with (the ACLU) early next week.”