Opponents were on one side of the street, while supporters took to the other side. Opponents carried signs associating Islam with blood, supporters shouting, "Say no to racist fear!" and American flags waved on both sides. For the most part, it was peaceful.
Both sides argued freedom of religion, while the protesters said "just not here."
As the furor raged on at home, the imam at the center of the discussion was far from the public debate, in Bahrain on a trip paid by U.S. taxpayers to promote understanding about Islam and the United States.
Faisel Rauf's audience was young and educated Arabs from Bahrain's capitol. They seemed impressed.
"Having an American imam, and he's a Muslim, it does really improve the image of America," one student said.
The imam says he hopes to build cultural and religious bridges with the mosque and cultural center. But how is that working?
"Racism and bigotry have found another face, and now it is against Muslims," one supporter said.
"Why put it so close if you want to bring people together?" countered protester Steve Morris. "Put it away from it, so that there's no pictures to show that this mosque is right where our children died."
Police were on the scene during the demonstrations, but no arrests were made.
It was by far the largest organized protest for and against the Park51 project.
The project is a $100 million, 13-story Islamic center slated to be built two blocks north of the former site of the twin towers.