Undocumented immigrants are applying for the temporary right to live and work openly in the US, as a sweeping immigration policy reform takes effect.
Up to 1.7 million people could be eligible for the programme, unveiled in June by President Barack Obama amid pressure from Hispanic voters.
Republicans say Mr Obama has passed over Congress - and unemployed US citizens - with the programme.
The Latino vote could be important in November's presidential election.
Most of the estimated 11.5 million undocumented immigrants in the US are from Latin America.
Illegal immigrants are getting their records in order as the Department of Homeland Security (DHS) starts accepting applications.
In an internal document, DHS officials estimated 1.04 million people would apply in the first year.
The Migration Policy Institute and the Pew Hispanic Center have estimated as many as 1.7 million people could be eligible under the programme.
The administration's plan is to stop deporting many illegal immigrants who were brought to the US as children.
To be eligible, immigrants must prove they arrived in the US before they turned 16, are 30 or younger, have been living here at least five years and are in school or graduated or served in the military.
They also cannot have been convicted of certain crimes.
The measure has echoes of the so-called Dream Act, which offered permanent residency to young undocumented immigrants, but failed to pass after years of congressional stalemate.
"Childhood arrivals who meet the guidelines and whose cases are deferred will now be able to live without fear of removal, and be able to more fully contribute their talents to our great nation," said Alejandro Mayorkas, head of US Citizenship and Immigration Services.
But conservatives accused Mr Obama of political pandering as he and Republican White House rival Mitt Romney compete for the Hispanic voting bloc ahead of November's elections.
Republican House Judiciary Committee Chairman Lamar Smith said on Tuesday: "While potentially millions of illegal immigrants will be permitted to compete with American workers for scarce jobs, there seems to be little if any mechanism in place for vetting fraudulent applications and documentation submitted by illegal immigrants."
The Obama administration detailed on Tuesday what documents illegal immigrants need to qualify for the programme.
The paperwork for the programme can be downloaded from the immigration services website.
Applicants must pay a $465 (£300) fee and provide proof of identity and eligibility.
This could include a passport or birth certificate, school transcripts, medical and financial records, military service records and, in some cases, multiple sworn affidavits.
A decision on each application could take several months, and immigrants have been warned not to leave the country while their paperwork is being processed.
Advocacy groups are planning to run events this week to help migrants get their paperwork in order.