By Lacey Johnson
(Reuters) - Maryland Governor Martin O'Malley on Thursday signed into law one of the United States' toughest gun control measures, even as opponents vowed to overturn it.
The legislation prompted by the Newtown, Connecticut, school massacre requires handgun buyers to undergo safety training and submit fingerprints to obtain a license.
It also bans the sale of 45 types of assault weapons, which have been linked to at least 461 U.S. deaths since 2004, according to the governor's office.
"We've chosen to take action by advancing the strategies that work to save lives," O'Malley, a Democrat mentioned as a potential presidential candidate in 2016, said in a statement.
The new law also reduces magazine capacity from 20 rounds to 10 rounds. People who have been involuntarily committed to a mental health facility will be banned from gun ownership.
Maryland joins Connecticut and New York in expanding assault weapons bans and restricting the capacity of magazines in the aftermath of the Newtown elementary school shooting that killed 20 students and six adults in December.
Colorado approved similar measures after a gunman killed 12 people and wounded 58 in July 2012 in a movie theater.
The National Rifle Association (NRA), the powerful gun lobby, has announced plans to challenge the new state gun control laws, including those in Maryland.
Republican State Assembly Delegate Neil Parrott said last month that leading opponents of the Maryland bill planned to back an NRA lawsuit, rather than petition for a referendum.
Senate President Thomas Miller criticized the NRA. He told a news conference in Annapolis that the NRA "is going to look for a conservative, right-wing-leaning judge, rather than going to the people of the state of Maryland ... because they know that if this bill went to the people, it would pass overwhelmingly."
A Washington Post poll in February showed that Maryland residents supported O'Malley's licensing plan. Eighty-five percent backed it, and 73 percent said they did so "strongly."
A Maryland woman, Sue Payne, has also launched a campaign to petition the bill to a referendum, filing paperwork with the board of elections last month.
Other features of the bill include mandatory reporting of lost or stolen firearms. It also strengthens the state's ability to regulate gun dealers and bars some violent offenders from obtaining a gun while on probation.
(Reporting by Lacey Johnson, writing by Ian Simpson; Editing by Paul Thomasch and Maureen Bavdek)