(CNN) -- Thousands of demonstrators gathered in U.S. cities Thursday to protest federal immigration raids and deportations and to call for comprehensive immigration reform.
Protesters in San Francisco, California, rally Thursday in support of amnesty for all illegal immigrants.
In Chicago, Illinois, 3,000 to 4,500 people marched in the city's downtown, police said. Several people carried a large American flag; others held banners or signs.
The early estimate of participants paled greatly in comparison to protests in Chicago in past years: In 2007, numbers reached about 150,000, and the year before, estimates ranged from 400,000 to 700,000.
In New York, hundreds of sign-carrying protesters gathered in Union Square, preparing for a march toward Foley Square in downtown Manhattan.
"We are demanding that the raids and deportations stop," said Teresa Gutierrez, one of the organizers for the New York rallies.
"We are for the rights of all immigrants, whether they're documented or not," she said.
New York protesters are also marching in support of workers' rights, she said. Watch Gutierrez explain the issues »
In Milwaukee, Wisconsin, thousands of protesters carrying flags and signs descended on the city for a scheduled march.
The march was scheduled to proceed from the city's southside to a park that overlooks Lake Michigan, said the Web site of Voces de la Frontera, the group organizing that city's march.
Last year, 80,000 to 95,000 people participated, the group said.
Turnout was even less impressive in the nation's capital. About a dozen people began protesting outside of the Republican national headquarters Thursday afternoon and later moved to the Democratic national headquarters.
Another 60 people gathered about noon by the U.S. Capitol's Reflecting Pool where, in the rain, some finished writing protest signs. A group of American Indians joined the demonstration, saying they are often mistaken for illegal immigrants.
Elsa Zambrano, among the protesters at the reflecting pool, said she wanted a path toward legalization for "the folks who are here, and to stop the raids. Children are left at bus stops not knowing their parents have been arrested. This is not just immigration; it's racial profiling."
Thursday marks the third year of such rallies and demonstrations on May 1, known as May Day.
However, 2008 is the first time the rallies have been held during an election year, and protesters are hoping to draw the candidates' attention to their cause.
The two main Democratic candidates, Sens. Hillary Clinton of New York and Barack Obama of Illinois, have supported legislation that the presumptive Republican presidential nominee, Sen. John McCain of Arizona, co-sponsored.
Among other things, the legislation would increase funding and border security technology, improve enforcement of existing laws and provide a legal path to citizenship for some illegal immigrants.
Critics had lambasted the legislation, which President Bush backed, as "amnesty." The measure has stalled in the Senate.
The three candidates have also supported the construction of a 700-mile fence along the U.S.-Mexican border.
"The debate might have died in Congress," Gutierrez said, but "immigration policy is being carried out every single day statewide, locally and nationally."
"We need to remember that [Immigration and Customs Enforcement] is carrying out immigration policy every day," she said. E-mail to a friend