Locked-out faculty at Long Island University’s Brooklyn campus picketed on Wednesday as some students said substitute professors seemed ineffectual or unqualified.

The replacements are “qualified faculty with advanced degrees and expertise,” including some from LIU Post, in Brookville, a spokeswoman said.

On the first day of fall semester, students walked past picketing professors who chanted labor slogans and inflated a giant rat. The protesters could not go inside, because over Labor Day weekend the university barred the unionized faculty from their offices, locked them out of their email accounts and froze their health insurance after the Aug. 31 contract expiration.

“If we’re one university, pay all the faculty the same wage,” said Jay Shuttleworth, a full-time assistant professor who was supposed to teach Curriculum in the Secondary Classroom later that night. Instead, he held a sign Wednesday morning lamenting being paid 16 percent less in Brooklyn than he would be as a professor at the LIU Post campus.

The university says the Brooklyn pay is fair and outpaces peer institutions and that the union’s pay proposals would jeopardize LIU’s ability to keep tuition down. Union president Jessica Rosenberg, an associate professor of social work, countered that faculty pay represents only a small portion of the university costs and points to a parity clause in the contract.

The union, the Long Island University Faculty Federation, has voted 226-10 to reject LIU’s latest contract proposal.

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Gale Stevens Haynes, the university counsel, said in a statement the lockout was authorized to prevent disruption, noting past strike votes. A strike, Haynes’ statement said, would mean “holding the university captive to union demands.”

Negotiations continue with the union, which represents hundreds of full-timers and adjuncts.

Senior Brittney Parillon, 23, of Flatbush, Brooklyn, said her physics class was taught not by the professor she signed up for, but by a substitute who she said worked for the school’s IT department.

“He’s not well-versed in the subject, and we’re paying a lot of money for someone who’s qualified,” said Parillon, who is taking out student loans to pay her $25,000 annual tuition.

Sharda Mohammed, 18, a sophomore philosophy major from Richmond Hill, Queens, said an after-class physics lab was canceled and the substitute for another class, music, didn’t hand out a syllabus, as is typical. He played the piano, talked about music, and showed the class videos of performers.

“He didn’t really have anything to teach,” Mohammed said, adding: “The class was pretty much useless.”

Asked about the students’ complaints, Haynes said through a spokeswoman’s email that the instructor of the physics class has a Ph.D. and has taught the subject for decades. The statement, which said all “temporary staff” were qualified, did not address the claims about the canceled lab or the music teacher.

Wednesday was an unusual introduction to higher education for freshmen like Rohan Persaud, 20, of Jamaica, Queens, who rushed across Flatbush Avenue to his first university class ever, math.

But before he crossed, Persaud stood on the median. He held up his cellphone to memorialize the marching teachers and the giant rodent signaling labor discord.

“It’s kind of weird coming to this on your first day,” he said.