Barack Obama, the US president who promised to make overhauling the immigration system a top priority in his first year, sought to reassure those at the rally with a video message presented on giant screens at the National Mall.
The president said he was committed to working with congress this year on a comprehensive bill to fix a "broken immigration system", identifying problems such as families being torn apart, employers gaming the system and police officers struggling to keep communities safe.
The president, whose comments were released as he worked to get last-minute votes on the healthcare overhaul, said he would do everything in his power to forge a bipartisan consensus on immigration reform, saying: "I've always pledge to be your partner as we work to fix our broken immigration system and that is a commitment that I reaffirm today."
But he warned that reform "won't happen overnight".
On Thursday, Obama embraced a framework for legislation offered by Lindsey Graham, a Republican senator, and Charles Schumer, a Democratic senator.
Obama said the plan, which features a new identification card for US citizens and legal immigrants who want a job, "can and should be the basis for moving us forward".
But on Friday, Graham said that "if the healthcare bill goes through this weekend, that will, in my view, pretty much kill any chance of immigration reform passing the senate this year".
He and other Republicans have complained about the methods used by Democrats to win support for the healthcare bill.
Immigration is a sensitive issue in the United States where an estimated 11 million illegal immigrants live and work in the shadows and where Hispanics, the largest immigrant group, are a rapidly increasing voting bloc.
Obama had promised to reform immigration laws during his campaign for the presidency, but a crush of other priorities has held up efforts to legalise immigrants and offer a path to citizenship.
Frustrated that Obama has yet to fulfil a pledge to overhaul the nation's immigration system, immigration advocates have warned him to deliver this year or face the consequences in congressional elections in November.
"A lot of these people are here like myself are voters and if they don't do something, then I am voting against that congressman," Gumecindo Salas, vice-president of government relations for the Hispanic Association of Colleges and Universities, told the Reuters news agency.
While the majority of Hispanics are seen as unlikely to switch support to Republicans, they could hurt Democrats by failing to turn out at the polls in November.