Crowds gathered as the small stage was being constructed near the park's fountain, though people didn't know who they were waiting for.
Madonna took the stage at 7:30 p.m. to reactions from a surprised crowd and announced the goal of the pop-up rally was peace and unification.
"This concert is about unifying us," she said. "How are we going to keep America great? We are going to elect a president who does not discriminate against women, who does not discriminate against the LGBT community, who does not discriminate against people of color, that does not want to build a wall around our country and separate us from the rest of the world. This is a concert about peace."
"Save this country," she urged the crowd, "vote for Hillary Clinton."