Leading US Republicans have denounced President Barack Obama's move to tighten gun controls.
House of Representatives Speaker Paul Ryan said the executive orders, which bypass Congress, "undermined liberty" and would be challenged in court.
Republican presidential frontrunner Donald Trump said that, if elected, he would reverse the measures.
In an emotional address on Tuesday, Mr Obama accused the gun lobby of holding the country hostage.
Wiping away tears, he recalled the 2012 Sandy Hook primary school shooting in which 20 children and six adults were killed.
President Obama's executive actions involve:
- Background checks for all gun sellers, overturning current exemptions to some online and gun show sellers
- States providing information on people disqualified from buying guns due to mental illness or domestic violence
- Increased workforce for the FBI to process background checks, hiring more than 230 new examiners
- Congress being asked to invest $500m (£339m) to improve access to mental healthcare in the US
- The departments of defence, justice and homeland security exploring "smart gun technology" to improve gun safety
He previously told the BBC that the failure to tackle gun control had been the greatest frustration of his presidency.
More on US gun debate
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Guns at home - the question parents hate to ask before a playdate
However, the largest gun lobby group, the National Rifle Association (NRA), said Mr Obama's steps would not have prevented any recent mass killings in the US.
"Rather than focus on criminals and terrorists, he goes after the most law-abiding of citizens," said Mr Ryan, a Republican.
"His words and actions amount to a form of intimidation that undermines liberty."
Correspondents say the announcement is already shaping up to be an issue in the 2016 presidential election.
Donald Trump said that he would "un-sign" the measures if elected to the White House - a sentiment echoed by other Republican presidential candidates.
Senator Ted Cruz tweeted that the executive actions were unconstitutional, with a link to sign up for his campaign correspondence on a web page that says "Obama wants your guns".
Former Florida governor Jeb Bush, also a Republican candidate, tweeted that he would repeal the actions and protect the Second Amendment.
Leading Democratic candidate Hillary Clinton tweeted: "@POTUS is right: We can protect the second Amendment while protecting our families and communities from gun violence. And we have to."
President Obama announced the law change at the White House, surrounded by survivors and relatives of victims of shootings.
"The gun lobby may be holding Congress hostage right now, but they can't hold America hostage," he said.
Gun violence is significantly higher in the US than in other advanced countries, killing about 30,000 people each year.
Sales of guns in the US appear to have risen recently amid speculation that the White House was going to tighten the law.
Shares in gunmaker Smith & Wesson rose to their highest value since 1999 ahead of President Obama's announcement.
Congress has been reluctant to pass any laws restricting gun ownership, facing pressure from gun owners and the NRA.