Barack Obama mounted the biggest challenge in two decades to America's deeply ingrained gun culture on Wednesday, announcing a sweeping set of proposals to ban automatic weapons, limit magazines to 10 bullets, introduce universal background checks for all firearms buyers and increase scrutiny of mental health patients.

At a carefully stage-managed White House press conference a month after the Sandy Hook school massacre, Obama said the US had waited too long to tackle gun violence and it was time to act.

Flanked by a group of schoolchildren, Obama said: "This is our first task as a society: keeping our children safe. This is how we will be judged, and their voices should compel us to change."

Obama said his reforms are intended to reduce not just these killing sprees but the everyday gun violence suffered across the US. In the month since Newtown, 900 Americans had been killed by guns, he said.

"In the days ahead, I intend to use whatever weight this office holds to make them a reality," he said.

The National Rifle Association immediately vowed all-out opposition to his proposals, warning it was preparing for "the fight of the century". Wayne LaPierre, the NRA executive vice-president, said: "I warned you this day was coming and now it's here. It's not about protecting your children. It's not about stopping crime. It's about banning your guns … period."

Obama, having abandoned any faint hopes of winning round the NRA after Newtown, has opted to confront the NRA head-on, calculating that the emotion over the deaths of 20 children and six teachers has changed the national mood in favour of tougher controls.

On stage with him at his press conference were four children who wrote to him after the shooting expressing concern. Also in the audience was a survivor of the Virginia Tech shooting and the parents of a child killed in Newtown.

In a particularly poignant part of his speech, Obama spoke about that child, Grace McDonnell, describing her favourite colour and ambitions, and reflected not only on her short life but the potential that had been lost. The president revealed that he has a painting by Grace, given to him by her parents, on the wall of his study.

The White House described Wednesday as "repugnant and cowardly" an NRA ad that brought Obama's two daughters into the debate.

But the president faces an enormous struggle to get his more ambitious proposals through Congress because of almost certain opposition from Republicans, who control the House, and some Democrats, especially in the Senate, which will start looking at the measure in the next two weeks.

House Republicans issued a non-committal response, saying simply that they would consider any legislation that comes to it from the Senate, where any bills would start and where the Democrats have a majority.

Michael Steel, a spokesman for the House Republicans, said: "House committees of jurisdiction will review these recommendations. And if the Senate passes a bill, we will also take a look at that."

But Reince Priebus, chairman of the Republican National Committee was more forthright, denouncing Obama's proposals as an "executive power grab that may please his political base but will not solve the problems at hand".

Democratic representative Carolyn McCarthy of New York, who was at the White House, said: "I have seen what a body looks like when someone is shot but this time it is different. What the president has done today is a start but there is a limit to what he can do and the rest is up to us in Congress to act on."

Obama acknowledged it will not be easy getting legislation through but said he had to try. "If there is even one thing we can do to reduce this violence, if even one life can be saved, we have an obligation to try," Obama said.

Immediately after his statement, Obama sat down to sign 23 executive orders that will take immediate effect and also set in motion various other actions that do not require Congressional approval.

These measure range from naming a new head of the Bureau of Alcohol, Tobacco, Firearms and Explosives – a post Congress has blocked for six years – to bypassing a congressional ban on research into any possible links between violence and video games. Other measures include providing more police on the streets and extra security for any school that requests it.

"These are in no way a substitute for action by Congress," Obama said. Significant changes will require legislation.

The most high-profile reform Obama proposed to put to Congress on Wednesday is a tougher version of the 1994 law introduced by Bill Clinton that banned automatic- and semi-automatic weapons but which was criticised by the gun control lobby as having too many loopholes.

Legislation will also be needed to reduce the number of bullets in magazines to 10, which pro-gun control activists argue could limit the number of casualties in shooting sprees.

But just as far-reaching would be the introduction of universal background checks for all gun sales. While federally licensed gun sellers are required to carry out checks on buyers, gun shows and online sales are exempt.

Legislation would end this loophole which allows, according to Obama, some 40% of all gun sales to take place without a background check.

In a direct challenge to the NRA and the House Republicans – as well as some of his own Democrats – Obama said that members of Congress should be more concerned about the safety of children heading off to school in the morning than about securing an A-rating from the NRA.

This is the first time that a president has attempted to tackle gun violence since 1994. In the intervening years, the gun lobby has regained ground, supported by the supreme court.

Groups in favour of tightening gun laws hailed the Obama announcement as a seminal moment. Josh Sugarmann of the Violence Policy Center said the proposed reforms amounted to the "broadest package of changes that has been put forward by any administration since the 1960s".

If the president succeeds in driving through his preferred legislative changes - notably the revised and strengthened ban on assault weapons, a limit to magazine sizes and background checks on all gun sales - he will have achieved a great deal, Sugarmann said. "It would ratchet down the firepower that is in civilian hands - anything that reduces that firepower is an important step forward."

Dan Gross, president of the Brady Campaign to Prevent Gun Violence, praised the White House for showing "tremendous leadership" and for instigating a conversation that "so many Americans have been calling for in the wake of Aurora, Newtown, and the 32 gun murders that happen every day in our country".

Sam Hoover of the Law Center to Prevent Gun Violence predicted a tough fight ahead, but welcomed broad action now at a time when the epidemic of gun violence was getting more serious in many parts of the US. "This should have been done years, if not decades ago, but we're pleased to see it finally happening."

In a statement, the NRA said "keeping our children and society safe remains our top priority". It continued: "Attacking firearms and ignoring children is not a solution to the crisis we face as a nation. Only honest, law-abiding gun owners will be affected and our children will remain vulnerable to the inevitability of more tragedy."