A Manhattan man went wild in Central Park yesterday, smashing a police van with a tree branch and swimming out to an island in the Harlem Meer, where he spent 90 minutes singing and shouting before cops — 15 of them — nabbed him.

Alionme Dieng, 25, began his rampage at 10:30 a.m., and stripped to his undies on the islet, witnesses said.

After ranting more, he dove back in to the water.

“He went in with shorts,” said witness James Miller, “but he didn’t come out with shorts.”