US President Barack Obama has said he will press for further immigration reform, after Republicans condemned his use of executive powers on the issue.
Mr Obama said he had no choice but to act, accusing the Republicans of blocking a bill in Congress, and vowed to make permanent reform a reality.
The current move means that more than four million illegal immigrants will be allowed to apply for work permits.
Republicans say it will encourage more people to arrive unlawfully.
Republicans have been weighing their response to Mr Obama's executive action, but the dilemma they face is how to oppose his immigration reforms without alienating Hispanic voters - whose support they will need in the next election, the BBC's David Willis in Washington reports.
Speaking in Las Vegas, where he first outlined his plans two years ago, President Obama said: "I will never give up."
He said: "We're going to keep on working with members of Congress to make permanent reform a reality.
- the only undocumented immigrant mentioned in the speech was Astrid Silva
- she was brought to the US aged four, with just her doll, a cross and the dress she wore
- now she is working on her third college degree
"But until that day comes, there are actions that I have the legal authority to take that will help make our immigration system more fair and more just, and this morning I began to take some of those actions.
"When members of Congress question my authority to make our immigration system work better, I have a simple answer - pass a bill."
Mr Obama said the moves that he had taken under his executive action meant that "not everybody will qualify".
"That's the truth. That's why we're still going to have to pass a bill... This is a first step, it's not the only step."
Mr Obama has insisted that his proposals, which are the biggest immigration reforms since the mid-1980s, do not amount to an amnesty.
Mr Obama added: "The bottom line is, mass amnesty would be unfair, but mass deportation would be both impossible and contrary to our country's character. That's not who we are.
"We didn't raise the Statue of Liberty with her back to the world. We did it with her light shining as a beacon to the world."
'King or emperor'
A bill was passed in the Senate last year, but the Republicans have been refusing to bring it to a vote in the House of Representatives. The party now has a majority in both chambers following the mid-term elections this month.
Earlier on Friday, Republican Speaker of the House John Boehner said Mr Obama's action had sabotaged any chance of enacting cross-party reforms and damaged the presidency itself.
He said the president had acted unilaterally "like a king or emperor" and not through a democratic process.
And he added: "The action by the president (Thursday) will only encourage more people to come here illegally. [It] also punishes those who have obeyed the law and waited their turn."
The Republican leader said Mr Obama's actions failed to take into account the wishes of the American people, adding it was not the first time the US leader had acted without the consent of Congress.
"All year long I have warned the president that by taking unilateral action on matters such as his healthcare law or by threatening action repeatedly on immigration, he was making it impossible to build the trust necessary to work together," Mr Boehner said.
The same day, House Republicans filed a federal suit over the legality of parts of Mr Obama's signature healthcare reform law, nicknamed Obamacare.
'Out of shadows'
Under Mr Obama's new immigration plan, undocumented parents of children who are US citizens or permanent legal residents will be able to apply for work permits lasting three years.
There are estimated to be 11 million illegal immigrants in the US.
More than four million of them are expected to benefit from the reform package forced through using executive action, which allow Mr Obama to bypass Congress.
Only parents who have lived in the US for five years will qualify.
Another part of the package will extend a programme that gives temporary legal status to people who arrived in the US as children.
Currently only those under the age of 30 who arrived before 2007 can apply for the programme, which was launched in 2012 and already covers roughly 1.2 million people.
Mr Obama has abolished the age limit and extended the cut-off point to 2010, potentially extending the programme to a further 300,000 people.
The US leader said his measures would allow illegal immigrants to "come out of the shadows and get right with the law".
The vast majority of illegal immigrants in the US come from Mexico, according to figures supplied by Homeland Security.
They made up 59% of undocumented people in 2012, followed by El Salvador (6%), Guatemala (5%), then Honduras and Philippines (3%).