A trial that had become a rallying point for many Occupy Wall Street activists ended on Monday with a jury finding a protester guilty of assaulting a police officer at Zuccotti Park in Manhattan in 2012.

A jury of eight women and four men took less than three hours to decide that the protester, Cecily McMillan, 25, was responsible for assaulting the officer, rejecting her contention that she had reacted instinctively when he grabbed her breast during a protest on St. Patrick’s Day. Ms. McMillan had said she could not distinctly recall what happened amid the chaos of the night.

After the foreman announced the verdict in State Supreme Court in Manhattan, the courtroom erupted in catcalls and derision from more than 50 of Ms. McMillan’s supporters. Several stood and shouted, “Shame, shame, shame.”

Their anger peaked when Justice Ronald A. Zweibel ordered Ms. McMillan to be held in jail until sentencing on May 19. The second-degree assault on a police officer is a felony that carries a penalty of up to seven years in prison.