Hundreds of people took part in rival protests over plans for an Islamic centre near Ground Zero in New York.
Police kept the banner-waving groups apart, although there were no reports of violence between supporters and opponents of the scheme.
Opponents chanted "No mosque, no way" as supporters responded with "Say no to racist fear".
Developers plan to convert an old building into a 13-storey Islamic community centre and mosque.
Feelings ran high during Sunday's protests in Manhattan, close to Ground Zero.
There were face-to-face confrontations as demonstrators shouted at each other across the barricades, the Associated Press reported.
Opponents want the centre to be moved further from the site where more than 2,700 people were killed in the September 11 terror attacks.
Steve Ayling, a 40-year-old plumber from Brooklyn, told AP: "They should put it in the Middle East." The people behind the mosque project were "the same people who took down the twin towers", he said.
But Dr Ali Akram, a 39-year-old doctor also from Brooklyn, said that scores of Muslims died in the twin towers. Opponents of the centre were "un-American," he said.
"They teach their children about the freedom of religion in America - but they don't practice what they preach."
The plan by a New York developer to build the centre and mosque about two blocks from the former World Trade Center site has been attacked by prominent Republican politicians and conservative pundits.
President Barack Obama has come under fire over his defence of the developer's right to build a mosque a few blocks from Ground Zero.