A school science project that went awry and caused smoke to billow from a student's backpack sent panicked B train riders scurrying for the exits Thursday in Brooklyn.

"This is not a bomb! This is not terror!" yelled Gregory Kats, 29, a computer engineering student at the New York City College of Technology.

Kats, 29, was heading home from school when wires on a device he had built short-circuited in his backpack, he told the Daily News afterward.

When white smoke began spiraling out of his backpack as the train neared the Seventh Ave. station in Park Slope, riders quickly became alarmed, he said.

"They were panicking, and I realized their fear," an apologetic Kats said at his Sheepshead Bay home.

They "started to jump out of the train" as soon as it stopped at the platform and the doors opened, Kats said.

He said he tried to disassemble the contraption on the platform even as he reassured riders, "Don't worry. This is my science project."

A police bomb squad was dispatched, and cops grabbed Kats and questioned him.

But Kats was released a couple of hours later with his science project and without being charged, cops said.

The device, which has a small battery and motor, mimics the internal operations of an elevator, Kats said.

"I'm very sorry for what happened," Kats said. "I hate to scare people. Next time, I'll be much more careful and keep my electronic projects at home."