LOS ANGELES -- Tens of thousands of marchers crossed downtown and rallied at Los Angeles City Hall Saturday to urge immigration reform. The event was the largest May Day rally in the nation.

Authorities unofficially estimated the turnout to be between 40,000 and 60,000 people -- significantly less than the anticipated 100,000 attendants.

Though the crowd was roughly half as large as police had projected, it was the largest May Day turnout since 2006, when anger over federal legislation that would have criminalized illegal immigrants and those who aid them brought out more than 1 million protesters nationwide.

The crowd was "very orderly" all day, according to Los Angeles police spokesman Sgt. Frank Preciado.

Shouting matches broke out between the marchers and a small group of counterprotesters, but none of them led to violence.

Only two arrests were reported. Undercover officers arrested one man who was allegedly writing his name on a light post. Another man was arrested for allegedly being drunk in public and challenging others to fight.

Police officers also reunited two children who had become separated from their parents during the march.

Singer Gloria Estefan kicked off the march Saturday morning.

Speaking in Spanish and English atop a flatbed truck, Estefan proclaimed the United States a nation of immigrants. She said immigrants are good, hard-working people, not criminals.

Cardinal Roger Mahony stood on the truck chanting in Spanish,"Si, se puede," or "Yes we can."

Throngs of people blew horns and waved American flags. Marchers wore T-shirts that read "Boycott Arizona" and "Legalize Arizona."

Organizers handed out T-shirts that read, "Legalize Arizona" and "Boycott Arizona." Marchers waved American flags, along withmany from other countries, including Mexico, the Philippines, Nicaragua and Guatemala.

At about 11 a.m., the protesters began moving up Broadway from Olympic Boulevard. Marchers will walk north on Broadway toward City Hall.

It took more than four hours for the crowd to pass Third Street en route towards City Hall, where a rally was held in the area between 1st and Temple streets.

Los Angeles Mayor Antonio Villaraigosa took the stage at the rally to loud cheers. Speaking in English and Spanish, he called Los Angeles a "bilingual city" and expressed strong support for immigrants' rights.

Afterward, Mahony took the microphone. "Everyone in God's eyes is legal," he said. "We are all standing with our immigrant brothers and sisters."

Rallies in more than 90 other cities drew thousands of people from New York to Phoenix.

Arizona's new immigration law, and the political movement behind it, were targeted by speakers addressing the crowd from the City Hall steps.

The law requires authorities in Arizona to question people they come in contact with about immigration status when they have a reasonable suspicion that those people may be in the country illegally.

Critics say the law amounts to "racial profiling" and violates a person's civil rights.

"While we recognize that we must enact 21st-Century legislation to secure our borders and ensure the safety of all our citizens, the law signed by the governor of Arizona simply does not do this," Mayor Villaraigosa said last week. "The Arizona law is not only misguided, it is unpatriotic and unconstitutional.

"I add my voice to strongly opposing this unpatriotic and un-American law. And I call upon our federal leaders to pass comprehensive immigration reform in its place," Villaraigosa said.

L.A. Police Chief Charlie Beck also said he does not support the Arizona law, saying the LAPD's Special Order 40 -- which prevents officers from contacting people solely to verify if they are legal residents -- has worked effectively for more than three decades.

"It is the mandate of this police department that we do not initiate police activity due to immigration status," Beck said.

"That is my mandate. That is how I will police this city as long as I am chief of police. That's been consistent with my predecessors all the way back to Chief Daryl Gates. We do that not only because it makes our job more efficient and more effective, but because it's the right thing to do."