Maryland Gov. Martin O'Malley (D) details proposed gun-control legislation Friday at the Maryland State House. (John Wagner/The Washington Post)

Maryland Gov. Martin O’Malley’s ambitious gun-control package would require citizens who move to Maryland to register guns purchased in other states, and will make it illegal for residents younger than 21 years old to purchase or own registered firearms or ammunition.

Those restrictions were among new details the governor and his staff released Friday as they put the final touches on a gun-control package that has gained national attention as one of the most aggressive outside of President Obama’s federal gun-control proposals in the wake of the massacre last month in Newtown, Conn.

O’Malley (D) has proposed a broad assault weapons ban; new licensing requirement for gun owners and a proposed expansion of reporting of mental health problems to get guns out of the hands of those considered dangerous to others. The details of the governor’s plan were first reported Monday in the Washington Post.

It was not part of O’Malley’s proposed legislation, but when asked by a reporter, O’Malley said that he was also open to discussions with lawmakers about stationing more armed police officers in Maryland public schools — the response to public outcry over mass shootings that has been pushed by the National Rifle Association.

Flanked by police chiefs from across the state, O’Malley on Friday called his gun-control package his top legislative priority for the 90-day session of the Maryland General Assembly. The governor has also proposed repealing the state’s death penalty.

O’Malley dismissed criticism from Republicans and some Democrats that his proposed licensing requirement would trample on Second Amendment rights, saying he believes he has broad public support to require every new Maryland gun owner to obtain a license to own a gun.

“It is a common-sense licensing requirement,” O’Malley said. “If you have to get a license to drive a car in Maryland . . . you should have to be licensed in order to operate a firearm.”

Stacy Mayer, O’Malley’s legislative secretary, said the governor has reached decisions this week on more specifics of that proposal.

Firearm licenses would be issued by the Maryland State Police, and to qualify, prospective gun owners would be required to submit to digital printing, complete a hands-on weapon-familiarization and gun-safety course, and undergo a more extensive background check.

The required training course would be at least eight hours long, and gun licenses would expire after five years.

To maintain legal possession of a regulated firearm, every new Maryland gun owner would have to reapply for a license in five-year increments. The restriction will not apply to the state’s current gun owners, or those who purchase shotguns and rifles.

Marylanders currently possess over 700,000 registered firearms, mostly handguns, and an estimated 500,000 rifles or shotguns. There are also over 48,000 registered assault weapons.

O’Malley repeatedly labeled the latter “military” weapons and said they have no place in society other than on a battlefield.

In response to questions, O’Malley also said he was open to stationing more police officers in public schools.

Currently, about one in five public schools in Maryland have so-called school resource officers (SROs), who sworn law enforcement officers assigned to schools based on agreements between local education and law enforcement entities. Usually the officers are armed and typically uniformed. (About 12 percent of schools in Montgomery County have SROs, and about 11 percent have them in Prince George’s County).

The governor said that program could expand, but cautioned that the answer is “not to give every teacher an Uzi and a gun belt.”

O’Malley’s budget calls for spending $25 million next year to study and to begin to tighten physical security at public schools, including automatically locking doors, shatterproof glass and buzzer-entrance systems.

On mental health, O’Malley’s package goes only a fraction of the distance of gun-control restrictions passed this week in New York. It would ban guns from those in legal guardianship, whether for mental illness or cognitive decline. It would also seek to make those who are involuntarily committed to psychiatric institutions forfeit their firearms if they are deemed a potential danger to others.

O’Malley acknowledged that an assault-weapons ban, like the federal one that expired in 2004, would be more effective on the national level, but said he felt Maryland has a responsibility to act regardless.

The threat of gun violence “will never go away,” O’Malley said, “but the actions we take may well prevent another tragedy like that in Newtown, Connecticut. . . . It is possible we can make a difference.”