Firearms sales are surging in the US in the wake of the Colorado massacre as buyers express fears that politicians may use the shootings to seek new restrictions on owning weapons.

In Colorado – the scene of Friday's shooting during the screening of a Batman film that killed 12 and injured dozens of others – gun sales jumped in the three days that followed. The state approved background checks for 2,887 people who wanted to purchase a firearm – 25% more than the average Friday to Sunday period in 2012 and 43% more than the same period the week before.

Dick Rutan, owner of Gunners Den in suburban Arvada, said requests for concealed-weapon training certification were "off the hook". His four-hour course in gun safety, required for certification for a concealed-weapons permit in Colorado, had drawn double the interest since Friday.

"What they're saying is they want to have a chance. They want to have the ability to protect themselves and their families if they are in a situation like what happened in the movie theatre," Rutan said.

Day-to-day gun sales frequently fluctuate, but the numbers also look strong outside Colorado.

In King county in Washington state, there were nearly twice as many requests for concealed pistol licences as a year ago. Florida recorded 2,386 background checks on Friday, up 14% from the week before. Sales in Oregon on Friday and Saturday were up 11% on the previous month. Four days of checks in California were up 10% month-on-month.

During the past decade, June and July have consistently been the slowest months for gun sales, according to FBI data.

Jay Wallace, who owns Adventure Outdoors in Smyrna, Georgia, found his sales on Saturday were up 300% from the same day a year ago – making it one of the best Saturdays his business has ever had. He said customers were often afraid when there's a gun-related tragedy that some lawmakers might try and push through an anti-gun agenda.

"We shouldn't let one sick individual make us forget and lose sight of freedoms in this country," Wallace said.

Authorities have said that the suspected Colorado shooter, James Holmes, methodically stockpiled weapons and explosives at work and home in recent months. They said he purchased thousands of rounds of ammunition and a shotgun, a semi-automatic rifle and two Glock pistols.

On Friday, clad in head-to-toe combat gear, he is said to have burst into a midnight showing of the latest Batman film, The Dark Knight Rises, tossed gas canisters into the crowd and opened fire. The shooting killed 12 people and wounded dozens of others.

Police in the Denver suburb of Aurora say Holmes also booby-trapped his apartment. Holmes remains in solitary confinement at a local jail.

The rise in gun sales reflects one of many anxieties created by the attack. Since the massacre, there have been reports of chaos at cinemas, apparently sparked by misunderstandings or careless words.

A confrontation with an intoxicated man in Arizona caused about 50 people to flee the cinema, authorities said. A southern California man was arrested after authorities say he made allusions to the Aurora massacre after the movie failed to start on time. In New Jersey, a showing of the film was cancelled after someone stood up during the movie, opened an emergency exit and then returned to their seat.

About 90 minutes into a Monday night showing in Santa Monica, California, shrieks from some girls sent about two dozen people sprinting for the exit. It turned out that a large man with a backpack was actually not a threat and was simply having a medical problem.

"This was nothing, and yet it startled us and rattled us so much," said moviegoer Paria Sadighi.

Nationally, the shootings have triggered a fierce debate over gun control and whether the government should have a role in reining in the ownership of firearms.

Gun sales often fluctuate according to news events, especially whenever people think the passage of more restrictive gun laws is imminent. Sales spiked following the election of President Barack Obama, when weapons enthusiasts expressed fear that the Democrats might curtail gun rights. FBI figures also show background checks for handgun sales jumped in Arizona following the shooting of congresswoman Gabrielle Giffords in 2011.

"It's not uncommon for us to see spikes in requests for concealed pistol licences when there's a significant gun-related tragedy," said Sergeant Cindi West of the King county sheriff's office.

Some Democrats in Congress cited the shooting as evidence of the need for tougher gun control laws – particularly a ban on high-capacity ammunition magazines. Congress, however, has not passed strict legislation in more than a decade, and leaders in Washington DC show no sign of bringing up such measures any time soon.