[WATCH] US gun control inaction must end: Obama
An emotional US President Barack Obama has unveiled new restrictions on gun purchases, saying the "constant excuses for inaction" have to stop.
Wiping away tears, the president recalled the Sandy Hook primary school shooting in 2012 where 20 children and six adults were killed.
His executive actions, without Congress approval, widen background checks on potential gun buyers.
But the National Rifle Association said it would fight Obama's measures.
And the leader of the House of Representatives, Republican Paul Ryan, said the plans were certain to be challenged in the courts.
"His words and actions amount to a form of intimidation that undermines liberty," he said.
Meanwhile, sales of guns in the US appear to have risen, amid speculation in recent weeks that the White House was going to tighten the law.
Earlier, the president announced the law change at the White House, while 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," Obama said.
ANALYSIS - ANTHONY ZURCHER, BBC NEWS NORTH AMERICA REPORTER
An emotional president employed all of his rhetorical skill to justify what are, in reality, executive actions that only modestly expand federal regulation of firearm sales.
Standing in a room filled with victims of gun violence, he explained that the murder of schoolchildren in Newtown, Connecticut, changed him - and that he hoped it would change the country.
Three years have passed since that school massacre, however, and the country hasn't changed. While some states have toughened their laws, others have expanded gun rights and the US Congress has taken no action.
So Obama did what he could, and wrapped the move in language that sounded more appropriate for a ceremony announcing the passage of sweeping legislation that, in today's political environment, has no chance of reaching his presidential desk.
And even this small move will likely be fiercely challenged in court, in Congress and at the ballot box by whichever Republican wins the nomination fight to replace him in 2017.
Gun violence is significantly higher in the US than in other advanced countries, killing about 30,000 people each year.
Congress has been reluctant to pass any laws restricting gun ownership, facing pressure from gun owners and the powerful National Rifle Association.
Obama tried to pass expanded background check legislation in 2012 after the shooting in Newtown, Connecticut, that left 20 children and six adults dead but it failed in Congress.
THE EXECUTIVE ACTIONS INCLUDE:
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
The announcement is already shaping up to be an issue in the 2016 presidential election.
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."
Republican candidate Senator Ted Cruz tweeted that the executive actions are unconstitutional, with a link to sign up for his campaign correspondence on a webpage that says "Obama wants your guns" with a photo of the president in an army jacket and hat.
Former Florida governor Jeb Bush tweeted that he would repeal the actions and protect the Second Amendment.
During Obama's speech, comedian Amy Schumer, cousin of New York Senator Chuck Schumer, was in the audience. Two women died in a shooting at a movie theatre in Louisiana during a showing of her movie Trainwreck.
Gabby Giffords, a former congresswoman who survived a shooting, was there as well, in addition to many relatives of victims and survivors of mass shooting.
Shares in gunmaker Smith & Wesson rose to their highest value since 1999 ahead of the president's announcement.
The number of background checks on potential buyers - a guide to future sales - has risen in the wake of mass shootings in the past.