Across the country, demonstrators took to the streets in a call for national immigration reform.
It was the fifth year in a row that nationwide immigration rallies were held on May 1, but this year -- in the wake of Arizona's passage of a controversial state law aimed at cracking down on illegal immigration -- emotions were especially raw.
"I want everybody to know that this law is absolutely ridiculous," said Felicia Ruiz, one of hundreds who gathered in downtown Phoenix near the Arizona state capitol. "[The law] is not going to solve anything."
In Washington, dozens were arrested peacefully for failing to disperse near the White House in a choreographed display of civil disobedience. In Chicago and Dallas, protestors filled city streets; in Los Angeles, site of the nation's largest gathering, a massive crowd inundated downtown.
"Everybody needs to be here in this country," said Los Angeles demonstrator Osmin Vasquez. "They need opportunities to go to school, to go work, to go everywhere."
The new Arizona law, which was passed last month, requires local police to question anyone they reasonably suspect is an illegal immigrant. It also makes harboring illegals a crime. Opponents say the law opens the door to rampant racial profiling.
"The law is unwise," said Pima County, Ariz., Sheriff Clarence Dupnik. "It's stupid and it's racist. From my point of view, it's a national embarrassment."
Already, lawsuits have been filed in federal court challenging the law. Activists have called for a boycott of Arizona goods and services. And the state has been lampooned by late-night comedians.
But supporters of the measure say the law is necessary, and they cite growing violence at the Arizona-Mexico border.
On Friday, a sheriff's deputy was wounded during a gunfight in a well-known trafficking corridor for drugs and illegal immigrants. Such incidents are a regular occurrence.
"With the illegal aliens come drug smuggling, human smuggling, street crime and kidnapping," said Republican State Rep. John Kavanagh, a supporter of the new law.
In fact, polls show most Arizonans and a slim majority of Americans support Arizona's new law. With other states considering similar measures, May Day demonstrators say national immigration reform is needed more than ever, even though bipartisan efforts to craft such legislation has stalled in Washington.
"Stopping brown people on their way to the grocery store is not going to make [Arizona] more safe," said marcher Stephanie Souter. "They've got problems with immigration, but they need to fix those issues and not scapegoat brown people in their state."