President Barack Obama on Friday suspended the threat of deportation against hundreds of thousands of immigrants who were brought to the United States as minors, delighting crucial Hispanic voters ahead of November's election.

"These are young people who study in our schools, they play in our neighborhoods... they are Americans in their heart, in their minds, in every single way but one... on paper," Obama said at the White House.

The scheme applies to people brought to the United States before the age of 16, who are currently under 30, are in school or have graduated from high school, or have served in the military and have not been convicted of a felony.

"Put yourself in their shoes, imagine you have done everything right your entire life... only to suddenly face the threat of deportation to a country you know nothing about," Obama said.

"This is not amnesty, this is not immunity... this is the right thing to do," said Obama, who, in an unusual scene, was heckled by a journalist from the conservative Daily Caller website during his remarks.

The affected youths will be able to apply for work permits, but not be granted permanent residence or put on a path towards citizenship. The move offers a two-year deferment of deportation proceedings that could be extended by a further two years on expiry.

Homeland Security Secretary Janet Napolitano said that only young people that had been living in the United States for five years and were no threat to national security would be eligible.

Obama's decision will go some way to enshrining the goals of the DREAM Act, legislation backed by the White House that could lead to young illegal immigrants gaining permanent residency.

The bill, opposed by Republican presidential candidate Mitt Romney and Capitol Hill conservatives, has repeatedly failed to pass Congress and become law.

Romney said that although the plight of young illegals was important, the action Obama took "makes it more difficult to reach that long-term solution."

"I'd like to see legislation that deals with this issue," he said, adding that he agreed with the approach of US Senator Marco Rubio of Florida.

Rubio, seen as a possible future Republican presidential candidate, is trying to walk a tightrope on immigration between a hawkish Republican party and a desire not to alienate Hispanic voters, the country's fastest growing racial minority group.

"By once again ignoring the constitution and going around Congress, this short-term policy will make it harder to find a balanced and responsible long-term one," Rubio said.

Romney's position on immigration reform, adopted to appeal to conservative Republican primary voters, could come back to haunt him among Hispanics in general election swing states like Colorado and Nevada.

Other Republicans accused Obama of overstepping his powers and of bypassing the collective will of elected lawmakers.

"Americans should be outraged that President Obama is planning to usurp the constitutional authority of the United States Congress and grant amnesty by edict to one million illegal aliens," Republican Representative Steve King said.

The DREAM Act's "amnesty program ... has been rejected by Congress, and yet President Obama has decided that he will move forward with it anyway."

Immigration reform groups welcomed Obama's decision.

"The president has given us a reason to believe in him," said Cesar Vargas, managing partner at DRM Capitol Group, which fights for DREAM legislation around the United States.

"We will ensure that people go out to vote to keep this executive order alive."

Mexican President Felipe Calderon welcomed Obama's decision. "It is just recognition to their contributions" to the United States, Calderon wrote on his official Twitter account.

There are some 12 million Mexicans living in the United States, only half of whom migrated legally, according to Mexican government estimates.

Officials said the measure could impact around 800,000 youthful illegal immigrants. The Pew Hispanic Center said up to 1.4 million children and young adults could benefit.

There are 11.5 million illegal immigrants living in the United States, mostly of Hispanic origin, and efforts to deal with their status have foundered over sharp political divisions.

In an NBC/Wall Street Journal poll last month, Obama led Romney among registered Hispanic voters, 61 to 27 percent.

Obama promised to work towards comprehensive immigration reform when he ran for office but has made little progress. Now the president is pledging to tackle the issue if he wins a second term.