Gun rights activists aided by the National Rifle Association are suing Massachusetts over its firearms laws, saying the state's assault weapons ban is preventing law-abiding residents from buying and possessing some of the most popular rifles in the country, as well as most standard-capacity magazines.

The lawsuit, filed Monday in federal court by the Gun Owners' Action League of Massachusetts and other groups, specifically targets the state's 1998 assault weapons ban, which mirrors a federal ban that expired in 2004. The lawsuit contends that millions of citizens in other states are allowed to lawfully possess the weapons.

Massachusetts has some of the strictest guns laws in the country. It also had the lowest gun death rate in 2015, according to data released by the federal Centers for Disease Control and Prevention, which found about three gun deaths per 100,000 residents.

The lawsuit seeks to invalidate Democratic Attorney General Maura Healey's enforcement notice to gun sellers and manufacturers in July clarifying what constitutes a "copy" or "duplicate" weapon under the state's assault weapons ban, including copies of the Colt AR-15 and the Kalashnikov AK-47.

Healey has said the new rule would not be enforced against gun owners who bought or sold those weapons before the notice. At the time, Healey cited mass shootings, including the attack at a gay nightclub in Orlando, Florida, that claimed the lives of 49 people, as one reason for the crackdown.

Healey estimated that 10,000 copycat assault weapons were sold in Massachusetts in 2015.

Healey said Tuesday during an appearance on WGBH-FM that she will vigorously defend the state's gun laws.

In September, two gunmakers filed lawsuits saying Healey is overstepping her authority by investigating possible safety violations with firearms manufactured by the companies.

Healey's actions have prompted a vocal backlash from gun rights activists.

"If there is anything positive about AG Healey's unprecedented and unilateral politically motived attack on lawful citizens, it is that our plight has been pushed onto the national civil rights stage," GOAL Executive Director Jim Wallace said in a statement announcing the lawsuit.

Wallace acknowledged "the assistance and guidance of the National Rifle Association" in filing the lawsuit.

"Together we are drawing a line in the sand where Massachusetts' gun control agenda tramples the fundamental individual right to defend oneself and family in the home," he said.

The lawsuit also names Republican Gov. Charlie Baker as a defendant in his capacity as governor.

An aide to Baker said Tuesday that the governor supports both the Second Amendment and Massachusetts' gun laws, including the ban on assault weapons — but added that the administration generally doesn't comment on pending lawsuits.