Thousands of illegal immigrants took their message to the nation's streets Tuesday, carrying American flags and signs to demand a path to citizenship from Congress.

Demonstrators hope the marches will push Congress to answer their calls for action for an estimated 12 million illegal immigrants currently living in the United States before the presidential primary season.

Click here to view photos from the immigration rallies around the United States.

Participants dressed in red, white and blue in cities from coast to coast began gathering with signs and American flags for events in major cities including New York, Los Angeles, Phoenix, Chicago and San Antonio on the May 1 International Workers' Day.

A few hundred people in Los Angeles County, home to an estimated 1 million illegal immigrants, showed up a few hours before a downtown Los Angeles rally was to begin.

March organizers expect fewer people this year in Los Angeles but the events will still cause traffic jams and disruptions to mass transit and businesses. Police are preparing for about 100,000 people, putting the department on maximum deployment with at least 1,000 officers.

"Everyone who can work will be working. Everyone who can be in uniform will be in uniform. All our detectives and sworn support personnel will be available to go out," LAPD Capt. Andy Smith told the Los Angeles Times.

• Click here to read the Los Angeles Times report.

Transportation officials are stationing California Highway Patrol officers at ramps near the march areas to deter marchers from taking their protests to freeways, warning of arrests.

March organizers say the events nationwide aim to target lawmakers.

"If we don't act, then both the Democratic and Republican parties can go back to their comfort zones and do nothing," said Angelica Salas, director of the Coalition for Humane Immigrant Rights of Los Angeles. "They won't have the courage to resolve a major situation for millions of people."

In Miami, Democratic Party Chair Howard Dean was scheduled to speak to a coalition of immigrant groups, while Ricardo Chavez, the brother of famed agricultural labor leader Cesar Chavez, was expected address crowds in Milwaukee.

In Washington, D.C., about 400 members of Asian groups from across the country were set to make a lobbying push with lawmakers. Students planned to march in Chicago.

Groups are planning an "American Family Tree" rally in New York, where immigrants will pin paper leaves on a large painting of a tree to symbolize the separation of families because of strict immigration laws.

The event is a response to a White House immigration reform proposal in March, said Chung-Wha Hong, executive director of the New York Immigration Coalition.

The plan would grant illegal immigrants three-year work visas for $3,500 but also require them to return home to apply for U.S. residency and pay a $10,000 fine. It has been roundly criticized by immigrant groups.

Democratic Party Chairman Howard Dean called the bill "insane" because it would require many illegal immigrants to return home before applying for citizenship.

Click here to view photos from the immigration rallies around the United States.

Organizers in Chicago rushed to alert a change of venue after police moved the site of their rally to Grant Park along the lakefront. Police said the original site downtown was too small for the expected crowd of more than 7,000 marchers.

"They announced the decision first and then they called us to consult us," said Jorge Mujica, a spokesman for Chicago's March 10 Movement group. "It doesn't make sense."

Nearly 500,000 people marched in Chicago last year.

Melissa Woo, a 22-year-old American citizen who immigrated from South Korea, carried a Korean flag over her shoulder as she criticized politicians for "buckling at the knees."

"Us immigrants aren't pieces of trash, we're human beings," she said. "To be treated as less than human is a travesty."

FOX News' Jeff Goldblatt and William La Jeunesse and The Associated Press contributed to this report.