With their expectations soaring, young illegal immigrants across the country are preparing to apply for a temporary reprieve from deportation that the Obama administration is offering. For the first time, as many as 1.7 million of them could be allowed to work legally and live openly in this country without fear of being expelled.

The program is President Obama’s most ambitious immigration initiative by far, a sweeping exercise of executive authority after Congress failed to pass the Dream Act, legislation he supported that would have given legal status to the young immigrants. It is a major bid by Mr. Obama to win back Latino voters who were souring on him after his administration deported nearly 1.2 million immigrants, most of them Latinos, in the last three years.

The initiative to defer deportations, which Mr. Obama announced in June, officially starts on Wednesday, when a federal immigration agency will begin accepting requests.

This weekend, the small offices of the Coalition for Humane Immigrant Rights of Los Angeles, a nonprofit group, were jammed with people seeking information about the program. Despite the baking heat, the lines stretched out the front door, to the end of a long city block. Similar crowds have flocked to immigrant and student organizations in other states for advice.

The program, which grants two-year deportation deferrals and work permits to illegal immigrants brought here as children, creates a herculean job for the federal agency in charge, United States Citizenship and Immigration Services. Agency officials are expecting the biggest load of paperwork in the shortest amount of time for a new program since 1986, when more than three million immigrants who were here illegally became legal residents under an amnesty.

Because deferrals are temporary and must be renewed after two years — when Mr. Obama may no longer occupy the White House — administration officials have been uncertain how many illegal immigrants would come forward to apply.

The work permit young immigrants can receive with the deferral opens many doors that have been firmly shut. They can obtain valid Social Security numbers and apply for driver’s licenses, professional certificates and financial aid for college.

On Saturday at the Los Angeles coalition, which is known as Chirla, many immigrants said they had calculated that the benefits would outweigh their doubts. All day, staff members offered hourlong presentations about the applications to groups of 200 or more immigrants, and still hundreds of people were turned away at closing time.

Although coalition counselors urged caution, many immigrants were not holding back their excitement.

“It’s like giving us wings to the people that want to fly,” said Noe Torres, now 26, who said he had been living illegally in California since his parents brought him here from Mexico when he was 4.

According to the Migration Policy Institute, a nonpartisan research organization, about 1.2 million foreign-born people are eligible to apply now, with another 500,000 children reaching the minimum eligibility age of 15 in coming years. By far the largest number — about 460,000 — live in California, with big populations in Florida, New York and Texas as well.

Immigrants who qualify for deferrals are generally the same ones who would have been eligible if the Dream Act had passed. But unlike that proposal (and the 1986 amnesty), the new program, formally called Deferred Action for Childhood Arrivals, suspends deportations but does not confer any legal status or open any future path to citizenship.

To be eligible, illegal immigrants must have been in this country and under age 31 on June 15, when the program was announced. They must have come to the United States before they were 16 and have lived here continuously for at least five years. They must be in school, or have graduated from high school or honorably discharged from the military.

An immigrant convicted of a felony, a serious misdemeanor (including a sexual abuse or drug violation), or three less serious misdemeanors will be rejected, as will anyone deemed to pose a threat to national security. There is no time limit for applications, which involve a $465 fee. In general, there is no appeal if an application is denied. But immigrants may apply again — and pay the fee again.

Homeland Security Department officials frame the program as part of the administration’s efforts to make good use of enforcement resources by focusing on deporting convicted criminals while avoiding noncriminals with strong family ties in the United States, particularly students.

To reassure wary parents who are here illegally, homeland security officials have said that application information will not be shared with enforcement agents.

Although the immigrants call themselves undocumented, they came to Chirla on Saturday with armfuls of documents tracing their time in the United States: school transcripts, awards for academic achievements and sports victories, high school and college diplomas, letters of recommendation, pay stubs, bank statements, rent checks, tax returns.

The advocates’ message was to avoid denials by taking time to get it right the first time. “We don’t want people to rush to apply,” said Angelica Salas, the coalition’s executive director.

But many immigrants see the program as a long-awaited release for pent-up academic and job aspirations. Cesar Cruz, 19, said he is a freshman at a community college and wants to earn a doctorate in bioengineering. He said his parents brought him here illegally from El Salvador when he was 10.

Mr. Cruz said he was eager for the day when he would give his fingerprints for the application. “I’ve been a good boy, you could say,” he said. “I want to help out my parents and show them their efforts weren’t for nothing.”

Hazar Parra, now 25, said she has been here since she was 7 and has just finished her training to be a nurse. A work permit would come just in time to allow her to apply for her state certificate.

Ms. Parra said she worried that her application might expose her mother, who is also here illegally. “My mom decided I should do it,” she said. “Basically, for me to do better and succeed, she’s willing to risk everything.”

White House and homeland security officials opted to entrust the huge effort to Citizenship and Immigration Services, the agency that issues immigration documents, rather than to enforcement agencies. The effort will be paid from fees, officials said, with no taxpayer dollars invested.

The agency has had a short time — 60 days — to prepare for the program, and its staff has been working long shifts. But its director, Alejandro Mayorkas, said it was ready to deliver “a positive and seamless customer experience.”

Still, until the applications start pouring in, Mr. Mayorkas has carefully resisted giving benchmark figures of how many officers will be dedicated to reading the applications, and how long it will take to approve or deny them.

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