Observers say it's no surprise Chicago provided the momentum, pulling off one of the city's largest marches in recent years with about three weeks' notice.

"Chicago seems to be for many reasons the political capital of immigration," said Jorge Mujica, one of the lead organizers of the March 10 rally. "There may be more immigrants in California or the border states, but Chicago is the place where lots of ideas came together."

Activists credit a large immigrant population, a network of established neighborhood groups and rising discontent over a proposed crackdown on illegal immigrants with galvanizing the community.

Chicago's rally grew from a mid-February summit in Southern California where civil rights groups from across the country discussed a national plan of action in response to the proposed crackdown, passed by the U.S. House in December.

The attendees set a March 10 target date for cities to hold massive rallies and marches. But organizers in Los Angeles and elsewhere weren't able to launch a huge event that quickly.

Chicago was the catalyst, according to Armando Navarro, coordinator of the Riverside, Calif.-based National Alliance for Human Rights, the umbrella group that organized the California summit.

"The only ones who moved on it aggressively was Chicago," Navarro said. "They took it, they ran with it and produced a political miracle. ... When I saw what happened in Chicago, I said, `My God, we're on track.'"

Groups in Chicago had already been meeting before December and had staged a smaller pro-immigration rally last summer. Roughly one in five Chicago-area residents is an immigrant, and more than 580,000 of the area's 1.4 million immigrants are from Mexico, according to a 2003 Roosevelt University study.

"This is the city of Jane Addams, of reformers and activists, and it has a rich immigrant history," said Antonio Zavala of the community group Casa Aztlan. "The city's foundation is an immigrant city, and that sense of community rubs off on everybody. We still have that sense of organization."

Dominic Pacyga, a history professor at Chicago's Columbia College, said the city's immigrant history - including Addams, a social work pioneer who ran a settlement home a community teeming with immigrants - has a legacy of organizing that has trickled down to the city's Latino immigrants.

"There always was an underground network in the immigrant community to get jobs and also to mobilize quickly," Pacyga said.

Chicago also is home to labor movements and political protests stretching back to the 19th century, including the 1886 Haymarket labor rally where a bomb killed seven police officers and led to the executions of anarchists unjustly convicted of the crime.

"It has always been a place that can put a lot of people on the streets quickly," Pacyga said. "There are public places where people can easily gather to show their anger and happiness. Chicago has always been that kind of place."

Once the March 10 rally idea was set, organizers used Web sites, e-mails and cell phones to spread the word quickly. They also posted signs in store windows, handed out fliers in churches and worked with radio deejays.

Rafael Pulido, known as "El Pistolero" on his talk show on WOJO-FM, was one of the deejays instrumental getting the word out. Though he's not a political radio host, the immigration issue kept his phone ringing and e-mail inbox filled with about 1,000 messages daily.

"How can you not do something when you have a microphone in front of you and you have a message?" Pulido asked.

Since Chicago's rally, immigrant-rights supporters have marched in cities across the country - including 500,000 in Los Angeles, at least at 350,000 in Dallas, about 50,000 in Denver and at least 10,000 in Milwaukee.

Organizers here say they're planning another march for May 1 to commemorate May Day, the international holiday honoring laborers. Though they're are unsure how many people will turn out, they say the issue is too important to not try for another mass demonstration.

"I haven't seen the spirits of the 1970s and 1960s until now," Zavala said. "This is really surprising. People are marching in the thousands everywhere. It indicates how deeply the immigrant and Mexican community in the United States feel about the attacks on immigrants."