Republicans have denounced President Barack Obama’s executive actions 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.

Donald Trump said that, if elected, he would reverse the measures.

In an emotional address on January 5, Barack Obama accused the gun lobby of holding the country hostage.

Wiping away tears, Barack Obama recalled the 2012 Sandy Hook primary school shooting in which 20 children and six adults were killed.

Barack 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 $500 million to improve access to mental healthcare in the US
• The departments of defense, justice and homeland security exploring “smart gun technology” to improve gun safety

The largest gun lobby group, the National Rifle Association (NRA), said Barack Obama’s steps would not have prevented any recent mass killings in the United States.

Correspondents say the announcement is already shaping up to be an issue in the 2016 presidential election.

Republican presidential hopeful 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”.

Jeb Bush, also a Republican candidate, tweeted that he would repeal the actions and protect the Second Amendment.

Barack 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 Barack Obama’s announcement.

Congress has been reluctant to pass any laws restricting gun ownership, facing pressure from gun owners and the NRA.