A man was arrested after pulling a BB gun on a fellow straphanger during a fight on a Brooklyn subway during rush-hour Wednesday morning, police said.

The tense incident happened around 9:05 a.m. as the northbound A train pulled into the Hoyt–Schermerhorn Streets A/C/G station in Boerum Hill, cops said.

The men on the train got into an argument and one of them reached into his fanny pack and pulled out what looked like a handgun, a police spokeswoman said.

An NYPD officer was on the train at the time, and he figured out that the weapon was actually a BB gun, cops said.

That officer and others who had been in the station took the BB gun-carrier into custody.

His identity wasn’t immediately released and charges were pending, cops said.