A 70-year-old man cut an elderly woman’s face with a knife after a brief dispute at a subway emergency exit door in Brooklyn on Saturday, as serious assaults in New York City’s transit system have spiked 12% over last year, according to police.

The confrontation broke out on the mezzanine level of the Sheepshead Bay B and Q station around 9 a.m., when the man, identified as 70-year-old retired postal worker Eric McCoy, was holding the exit door open, had a run-in with the 68-year-old victim, cops said.

The attacker then held a knife against the woman’s face, cutting her and causing her to bleed, a police source said.

The maniac fled on the southbound Q train, cops said. A person of interest was later taken into custody, and charges were pending, the source said.

The woman refused medical attention at the scene.

People sometimes hold the emergency door open and ask incoming riders for money to enter.

Felony assault in the city’s transit system is up 12% — to 525 from 470 — and misdemeanor assault is up 16% — to 1,304 from 1,120 — so far this year over the same period last year as of Oct. 13, according to the most recent NYPD data available.

Felony assault is also up 18% from two years ago and 94% from seven years ago, and the more minor misdemeanor assaults are up 11% over two years ago and 25% over seven years ago, according to the NYPD data, which doesn’t provide numbers for the older periods.

Transit crime is up 2% overall, and arrests in transit are down 5% so far this year to 22,847 from 23,977 at the same time last year, according to the data.