The turnout was relatively small, with rally crowds of a few dozen in many cities outnumbered almost 10 to 1 by counter-demonstrators who tried to drown out their voices with drums, bullhorns and cowbells.
ACT, which has drawn condemnation from civil rights groups for its frequent criticism of Islam and its efforts to pass state-level bills targeting Islamic law and refugees, organized the protests as a nationwide March Against Sharia and a defense of human rights.
"We're here protecting their rights, and they're trying to shut us down!" Pax Hart, the organizer of the New York City rally, told his audience, referring to the gathering of hundreds of leftist activists across the street. "It's insane!"
ACT for America, which the Southern Poverty Law Center recognizes as a hate group, was founded in 2007. Its leaders, who claim a 500,000-strong membership nationally, have labeled Islam a "cancer," propagated theories of a secret plot by Muslims, Democrats, communists and the media to destroy the country from within, and sponsored lectures on how to monitor and oppose U.S. mosques.
The group's founder, Brigitte Gabriel, has said that she is anti-sharia, not anti-Muslim, a point that a number of the group's speakers repeated Saturday. But Gabriel also has said that all practicing Muslims adhere to sharia, and speakers on Saturday made sweeping statements about Islam as an enemy of the state.
"We understand what Islam is, and we say 'no,' " Pawl Bazile, a member of the right-wing Proud Boys group, told a cheering crowd of a few dozen people in downtown Manhattan. "You're in the land of Budweiser and bikinis, for God's sake." Anyone who does not like it can move to Saudi Arabia or Syria, he said.
At least two white supremacist groups also joined the rallies.
In New York, a dozen members of Identity Evropa, which seeks a whites-only state, came to support the ACT rally, wearing tucked-in dress shirts, sunglasses and slicked-down side-parts. In Harrisburg, Pa., a group that has claimed credit for white nationalist posters on college campuses, said they wanted Muslims out of the United States entirely.
"I don't believe in having Muslims in the United States," said Francisco Rivera, a spokesman for Vanguard America. "Their culture is incompatible with ours."
Protesters on both sides blamed each other for the divisions on display Saturday, as anger surged through the crowds.
In New York, the cacophony from counter-demonstrators made it nearly impossible for them to hear their opponents' speeches. But many seemed to have already made up their minds.
"They're Nazis," said Krish Bhatt, a Barnard College student who held a sign identifying themself as a trans Muslim.
Organized in part to memorialize the anniversary of the Pulse nightclub shooting in Orlando, the multi-city demonstrations aimed to raise awareness of what ACT sees as the negative effects of Muslim immigration to the United States. The rallies were laden with many false and exaggerated claims.
"There's rampant rape happening because of Syrian immigrants, and we have to stop that from coming to America," said Joseph Weidknecht, a 25-year-old construction worker who attended a march in Austin.
In Harrisburg, Steven R. Moore, 35, said he joined the march because the Islamic State extremist group is trying to impose sharia in America.
Islamic scholars describe sharia, or Islamic law, as a set of guidelines for Muslim life, including finances, daily rituals, marriage and divorce. Sharia has a range of interpretations and practices, but certain guidelines for extreme types of criminal punishment, such as beheadings, tend to be cited by ACT as evidence of Islam's extremism.
Muslim leaders have countered that such extreme practices are similar to those endorsed in traditional readings of the Bible and Jewish law, and that few U.S. Muslims support such ideas.
In rallies in San Bernardino, Calif., New York and Seattle, columns of police moved in multiple times to separate rival protest groups as they shouted expletives at each other. In New York, masked anarchists tried on multiple occasions to get past the police cordon into ACT for America's rally, prompting one arrest, as ACT's speakers blasted the counterprotest as violent "idiots," "liars" and "Marxists," from a stage adorned with the American flag.
ACT protesters in San Bernardino yelled profanities as they rushed a group of counterprotesters shortly before fists began to fly. That rally had a few hundred supporters in a city affected by a 2015 terrorist attack that left 14 dead. In Seattle, police arrested two people and used pepper spray to end a scuffle between opposing sides.
Roughly 200 counter-demonstrators marched toward Seattle City Hall, where they significantly outnumbered those gathered for the anti-sharia law rally.
Several homeless people, many of whom live in the neighborhood where the rally was held, cheered as the demonstrators marched by chanting, "Muslims are welcome here, no hate, no fear" and "When Muslim rights are under attack, what do we do? Stand up, fight back."
Lisa Jaffe, 52, who carried a sign saying "loving my Muslim neighbor since 1964," said she participated in the counter-demonstration because the growing lack of tolerance has no place in her America.
Earlier in the day, In front of the capitol building in Harrisburg, anti-fascist protesters — wearing all black and balaclavas — played drums and cowbells, chanting "no hate, no Nazis." Separated by a police barricade, the anti-sharia protesters, some of whom were also masked and carried handguns, sang "America the Beautiful."
"This is a march against sharia, not Muslims," said Moore, of Washington County, Pa., who works in the oil and gas industry. "We are not affiliated with any extremist groups. . . . Sharia is a barbaric system that the Islamic State is trying to impose in our country."
Timmy Wylie, a spokesman for East Shore Antifa, said the group showed up to shut down the anti-sharia march because they say it is a march against Muslims. He grew up in the Harrisburg area and said citizens should instead focus on the region's struggling economy.
"There's a lot of people without two nickels to scrape together, but we still take care of each other," he said before the march.
Cat Cardenas in Austin, William Dauber in San Bernardino, Calif., Wiley Hall in Orlando and Lornet Turnbull in Seattle contributed to this report. Hauslohner reported from New York, and Moyer reported from Harrisburg, Pa.