The National Rifle Association, one of the most powerful lobbying groups in the US, has called for armed security guards to be posted in every school in the country and insisted that the only solution to gun violence in the wake of the Newtown massacre was more guns.

A week almost to the hour after a gunman blasted his way into Sandy Hook elementary school in Connecticut, killing 20 first-grade children as well as six staff members, the NRA's executive vice-president Wayne LaPierre finally broke his silence and delivered a defiant message to the nation.

Throwing down the gauntlet to Barack Obama, he declared: "The only way to stop a bad guy with a gun is a good guy with a gun." That is a mantra that he has used after several previous mass shootings.

His statement dashed hopes of gun control advocates that the NRA would be willing to engage in debate about tighter restrictions on gun ownership, such as a ban on assault weapons and large capacity magazines of the type used by Adam Lanza in Newtown. On Tuesday, an NRA statement promised the organisation would make a "meaningful contribution" to prevent mass shootings.

Tom Mauser, who lost his son Daniel in the Columbine massacre, said that he was disappointed but not surprised by LaPierre's comments. "He conceded nothing and completely ignored the desire of a lot of Americans for change – for the NRA change means just more guns."

LaPierre's comments set the scene for what could become a defining battle of Obama's second term in office. The president has already indicated that he means to use all powers at his disposal to effect meaningful change in America's relationship with guns, and has appointed his vice president Joe Biden to lead a national taskforce on the issue.

Over the past 20 years the NRA has proven to be a formidable foe of advocates of greater gun controls. By mobilising its army of three to four million members, backed up by a fearsome lobbying operation on Capitol Hill, the organisation has succeeded in blocking or watering down most previous attempts at tightening the country's uniquely lapse gun laws.

The NRA chief's unbending response to Newtown was delivered at a packed press conference in Washington that was disrupted twice by hecklers carrying banners that said "NRA: Killing Our Kids" and "NRA: Blood On Its Hands". In the course of about half an hour, LaPierre laid blame for the Sandy Hook massacre on several other parties including the media, politicians in favour of gun-free zones, the country's mental health services, gamers and the film studios that make violent movies – but brooked no criticism of the NRA itself.

He warned that there were "dozens, maybe more than 100 … monsters" out there already planning the next attack on an unprotected school. The only way to stop another gun rampage was to put guns in schools.

He said: "If we truly cherish our kids, more than money, more than our celebrities, we must must give them the greatest level of protection possible and the security that is only available with a properly trained – armed – good guy."

If Lanza – who also killed his mother last Friday before the attack on the school – had been confronted by a qualified armed security guard as he began his shooting spree, LaPierre ponderedI "isn't it at least possible that 26 little kids might have been spared that day?"

While other elements of the conservative movement in America have waxed and waned over the past two decades – with both the evangelical right and the Tea party suffering setbacks in recent times – the NRA has managed to sustain its impact on the national debate despite the on-going carnage of gun violence that claims about 12,000 lives every year.

But instant reaction to LaPierre's speech from conservatives suggested that this time the lobbying group's leadership may have overreached itself. "I don't necessarily think having an armed guard outside every classroom is conducive to a positive learning environment," the Republican governor of New Jersey Chris Christie told The Record newspaper.

Michael Steele, former chairman of the Republican National Committee, told MSNBC that the speech was "very disturbing. The idea that the message is 'Let's put a gun in the hands of teachers in our classroom' – I do not think that's where rank-and-file NRA members expected this to go."

Polling by the Republican analyst Frank Luntz has shown that there is considerable support for tightened gun laws even among NRA members. Some 74% would like to see criminal background checks for everyone buying a gun, in contrast to current laws that allow private sellers dealing through gun shows and online to by-pass such safeguards.

Leading advocates of greater gun controls also lambasted the NRA chief. Michael Bloomberg, New York's mayor who has been a crusader against gun violence, called LaPierre's comments "a shameful evasion of the crisis facing our country. Instead of offering solutions to a problem they have helped create, they offered a paranoid, dystopian vision of a more dangerous and violent America where everyone is armed and no place is safe."

Daniel Gross, the president of the Brady Campaign to Prevent Gun Violence, bemoaned that "what was said today is not indicative of the conversation the American public wants to have".

The Violence Policy Center pointed out that there had been two armed agents present at Columbine high school in Colorado during the 1999 assault that left 15 dead.

During the half-hour that LaPierre was in front of cameras two other events underlined the gravity of the debate that is now gripping America. The first was a minute's silence observed by the White House in respect for the dead of Newtown.

The second was a series of shootings that unfolded simultaneously on a quiet rural road outside Geeseytown in Pennsylvania. By the time the NRA chief stopped speaking, four more people had been added to the list of dead.