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.
Illegal immigrants are getting their records in order as the Department of Homeland Security (DHS) starts accepting applications for the programme, which would allow them to remain and work for at least two years.
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.
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.