Caroline O'Connor hops over a sign that will be used in a rally today in Los Angeles. DAMIAN DOVARGANES / THE ASSOCIATED PRESS

CHICAGO - Immigrant-rights activists hope Arizona's controversial immigration law will spark enormous crowds of people to protest in rallies nationwide and add urgency to pleas for federal immigration reform.

Dozens of marches are planned for today in cities across the country from Los Angeles to Dallas to New York.

"What happened in Arizona proves that racism and anti-immigrant hysteria across the country still exists. We need to continue to fight," said Lee Siu Hin, a coordinator with the Washington, D.C.-based National Immigrant Solidarity Network.

Activists believe opposition to Arizona's new law - which requires authorities to question people about immigration status if there's reason to suspect they're in the country illegally - could be the catalyst needed to draw record-breaking crowds similar to those four years ago.

That's when more than a million people across the country united to fight federal legislation considered anti-immigrant. Though the bill, which would have made being an illegal immigrant a felony, was unsuccessful, it triggered massive marches across the nation.

Since then, the movement has fractured and attendance has dropped sharply as attempts to reform federal immigration policy fizzled. In 2006, nearly half a million people took to Chicago's streets. Last year, fewer than 15,000 participated in the rallies, held annually on May 1 because it's a traditional day of protest and International Workers' Day.

But after the Arizona law was signed into law last week, immigration reform advocates have seen a flurry of activity.

Relying on online social networking, churches and ethnic media to mobilize, activists have called for a boycott of Arizona businesses and protested outside Arizona Diamondbacks baseball games.

Earlier in the week, two dozen activists chanting "Illinois is not Arizona" were arrested for blocking traffic outside an Immigration and Customs Enforcement facility in suburban Chicago.

While supporters say the law is necessary because of the federal government's failure to secure the border and growing anxiety over crime related to illegal immigration, critics say it's unconstitutional and encourages racial profiling and discrimination against immigrants or anyone thought to be an immigrant.

Activists fear that without federal legislation in place to address the estimated 12 million illegal immigrants living in the U.S., other states will follow Arizona's lead and pass similar legislation.

"If Republicans and Democrats do not take care of this albatross around our necks, this will in fact be the undoing of many, many years of civil-rights struggle in this country," said Jorge-Mario Cabrera, a spokesman for the Coalition for Humane Immigrant Rights of Los Angeles, where a downtown march is planned on Saturday. "I'm hoping that there is enough fire in people's hearts and minds to urge them to be mobilized."

But the chances that the federal government will step in this year with a comprehensive immigration reform bill seem slim.

President Obama once promised to tackle immigration reform in his first 100 days, but has pushed back that timetable several times. He said this week that Congress may lack the "appetite" to take on immigration after going through a tough legislative year.

However, the president and Congress could address related issues, like boosting personnel and resources for border security, in spending bills this year.

Meanwhile, activists say problems with a broken immigration system continue to affect millions - raids on workplaces create mistrust of authorities and separate families with mixed immigration status, employers take advantage of immigrant labor, and thousands of college students are left in limbo.