LOS ANGELES, May 1 (Xinhua) -- Los Angeles police and transit officials are bracing for immigration rallies that are expected to draw thousands of protestors in downtown on Tuesday.

    Beginning at 8:00 a.m. local time, protestors are beginning to gather at Olympic Boulevard and Broadway in downtown Los Angeles. At noon, the crowd will march north to City Hall, likely snarling access to the courts buildings and other government offices.

    Organizers said a second march will start at 2:00 p.m. at the intersection of West Third Street and Vermont Avenue in Koreatown, and head east and south along the sidewalks to MacArthur Park for a 5:30 p.m. rally.

    Sixty bus routes will be rerouted and parts of 17 downtown streets will be closed for much of the day as a result of the marches, city officials said.

    Mayor Antonio Villaraigosa is urging students to stay in school today and learn how to debate the issue in classrooms -- not in the streets.

    In the event students do participate in the downtown rallies, the city is prepared to send buses to pick up the children and bring them back to school, according to officials.

    Cardinal Roger Mahony, a vocal supporter of undocumented immigrants, urged students to voice their opinions by writing to their representatives in Washington, D.C.

    "Wouldn't it be fantastic if tomorrow 100,000 letters got mailed across this city from young people to our legislators in the Congress and Senate saying we desperately need immigration reform," Mahony said.

    The goal of the rallies, is to gain full legalization for the estimated 13 million illegal immigrants in America, according to organizers with the March 25th Coalition.

    Immigration activists have called for immigration legislation that would provide a path to citizenship for undocumented workers, and denounced a bill that would have made illegal immigration a felony.

    An estimated 400,000 to 500,000 people rallied on May 1 last year, demanding rights for undocumented immigrants.

    City officials do not know how many people will show up for today's rallies. Organizers have estimated 100,000 people, but Villaraigosa said he expects only about 20,000 protestors to turn up in downtown.

    "I'm not at this point expecting great numbers like last time," Villaraigosa said. "I don't see the same level of organization around this."

    Ira Mehlman of the Federation for American Immigration Reform, a nonprofit that seeks to improve border security and stop illegal immigration, said last year's demonstration did not build the kind of support organizers had anticipated.

    "Certainly it seems that the momentum has dissipated. It doesn't quite have the draw that it had last year," Mehlman said. "Number one, I think people last year thought that (the organizers)were in control and this was going to be a slam dunk and it turned out to not be so easy."

    "The American public also felt like they were being extorted," he said.