Organizers refused to cancel Friday's event because concerns over the H1N1 flu.
Similar events were held on Friday in Los Angeles, New York, Houston and other cities in hopes of influencing members of congress to change immigration policy.
Organizers chose Friday for their immigration reform march and rally for two reasons. May Day is a traditional day when workers rights are celebrated. And supporters thought it was time to place the issue of citizenship back into the forefront.
While some call Friday's event a success, others question whether it was as effective as in years past.
Protestors were demanding amnesty and justice.
"I think its necessary to be here to rise awareness about immigration here," said Estephany Campos, March participant.
Friday thousands marched for immigration reform despite the city's concerns about the spread of the H1N1 flu virus. Organizers refused to postpone or cancel the event, calling the request political.
"We don't see imminent danger so officers are takingthe same precautions like any other type of flu," said Commander Mike McCorter, Chicago Police Department.
It was hoped some 15,000 would participate like last year. Instead the number was closer to 2,500.
"We had a million people three years ago, but we had a million more voters on November 4," said Jorge Mujica, May Day march organizer.
The May Day rally comes as conservatives continue to blame the undocumented for everything from spreading the swine flu in the US to contributing to the country's economic downtown.
"There has to be enforcement on the border and enforcement of visas," said Michelle Jacobsen, Freedom Folks Blog.
Some here are angry President Obama hasn't addressed the issue of immigration.
"What we would like to hear Obama say yes to immigration reform in 2009. he didn't he skipped around the issue," said John Viramontes, rally participant.
And although Melissa Valladares has been here for 20 years illegally, she considers herself an American.
"No human being should be treated like a second class citizen," said Melissa Vallandares.
Regardless of the turnout, organizers call the event a success and say now supporters can truly rally around a unified immigration reform bill that could get pushed through Congress by the end of the year.