These qualifications resemble in some ways those of the so-called Dream Act, a measure blocked by Congress in 2010 that was geared to establish a path toward citizenship for certain young illegal immigrants. The administration's action on Friday, which stops deportations but does not offer citizenship or even permanent legal status, is being undertaken by executive order and does not require legislation.
What the younger immigrants will obtain, officials said, is the ability to apply for a two-year "deferred action" that effectively removes the threat of deportation for up to two years, with repeated extensions. "This is not immunity, it is not amnesty," said Janet Napolitano, the homeland security secretary. "It is an exercise of discretion."
Officials estimated that the new policy would cover about 800,000 people.
People whose deferrals are approved will then be able to apply for work permits, which will be dealt with case by case, an official said.
The decision highlighted the importance of Latino voters to Mr. Obama’s re-election campaign. Many of the states in which the election will be decided — Florida, Colorado, Virginia and Nevada among them — have large and growing Hispanic populations.
Mr. Obama’s action falls short of what some advocates have been seeking from an overhaul of the immigration system, and some Hispanic leaders have expressed disappointment that this president has not done more to address their concerns.
But the new policy represents a sharp contrast to the tone of the Republican candidates for president during the primary season, when Mitt Romney, now the Republican nominee, opposed the Dream Act and took a hard line against illegal immigration.
Mr. Romney has sought to build support among Hispanics mostly by emphasizing jobs and other economic issues over immigration. Mr. Obama’s new policy could put pressure on Mr. Romney to address the situation of young people who were brought to the United States illegally and have deep ties to their communities. But it has also given Republicans a chance to portray the president as acting in a blatantly political way in search of votes at a time when his campaign is being weighed down by slow job growth.
Senator Marco Rubio, Republican of Florida, called the administration's approach "a short-term answer to a long-term problem."
“There is broad support for the idea that we should figure out a way to help kids who are undocumented through no fault of their own," Mr. Rubio said, "but there is also broad consensus that it should be done in a way that does not encourage illegal immigration in the future. This is a difficult balance to strike, one that this new policy, imposed by executive order, will make harder to achieve in the long run."
Senator Richard J. Durbin of Illinois, a Democratic leader who with other senators had been seeking such an action for two years, called it "perfectly appropriate and legal."
Mr. Obama, under the mantra "we can't wait," has made a practice of pushing parts of his agenda forward by executive order when he cannot get action from Congress. But as a matter of executive power, officials said, this new policy could be reversed by the fiat of a future administration.
Details of the action were first reported by The Associated Press.
President Obama was expected to discuss the new policy in the White House Rose Garden Friday afternoon. As recently as Thursday, the president said that he would like Congress to go even further, giving some such people a path to citizenship.
"If we truly want to make this country a destination for talent and ingenuity from all over the world, we won’t deport hardworking, responsible young immigrants who have grown up here or received advanced degrees here," he said in a speech in Cleveland. "We’ll let them earn the chance to become American citizens so they can grow our economy and start new businesses right here instead of someplace else."
Immigrants have been beseeching the administration for just such an opening. Even so, since people who come forward would not obtain permanent lawful status, and the policy might be changed in the years ahead, it is uncertain whether everyone who qualifies will flood to the immigration service.
"People can make their own decisions about whether or not their circumstances warrant coming forward," an official said on Friday. The main incentive to do so, she said, was that it would make it easier to obtain a work permit.
Ms. Napolitano said that it made no sense to focus immigration enforcement on people who pose little if any threat to the nation.
"Our nation’s immigration laws must be enforced in a firm and sensible manner," said Ms. Napolitano. "But they are not designed to be blindly enforced without consideration given to the individual circumstances of each case. Nor are they designed to remove productive young people to countries where they may not have lived or even speak the language. Discretion, which is used in so many other areas, is especially justified here."
In its announcement, the Department of Homeland Security said it "continues to focus its enforcement resources on the removal of individuals who pose a national security or public safety risk, including immigrants convicted of crimes, violent criminals, felons, and repeat immigration law offenders. Today’s action further enhances the department’s ability to focus on these priority removals."
Richard W. Stevenson contributed reporting.