WASHINGTON - Loaded guns will be allowed in Yellowstone, the Grand Canyon and other national parks under a new law that takes effect Monday.

The law lets licensed gun owners bring firearms into national parks and wildlife refuges as long as they are allowed by state law. It comes over the objections of gun-control advocates who fear it will lead to increased violence in national parks.

The national parks law takes effect in a climate that favors advocates of gun rights. The debate shifted dramatically in 2008, when the Supreme Court struck down a handgun ban in Washington, D.C., and declared that individuals have a constitutional right to possess firearms for self-defense and other purposes.

Gun owners have rushed in record numbers to get concealed weapons permits, saying they worry President Barack Obama and the Democratic Congress may impose stricter gun laws. The National Rifle Association lobbied hard to allow guns in parks and has spent millions to challenge its opponents.

Now gun-control advocates are on the defensive, seeking to preserve some gun restrictions in the face of aggressive assertions of gun rights.

As of Monday, guns will be allowed in all but about 20 of the park service's 392 locations, including some of its most iconic parks: Yellowstone, the Grand Canyon, Great Smoky Mountains, Yosemite and Rocky Mountain National Park. Guns will not be allowed in visitor centers or rangers' offices, because firearms are banned in federal buildings, but they could be carried into private lodges or concession stands, depending on state laws.

Paul Helmke, president of the Brady Campaign to Prevent Gun Violence, said national parks are now among the safest places in America, but that could change under the new law.

Current rules severely restrict guns in the national parks, generally requiring them to be locked or stored.

"It really is sad that we've become such a paranoid society that people want to take guns pretty much everywhere - including national parks," he said Friday.

"When you are at a campfire and people are getting loud and boisterous next to you, you used to have to worry about them quieting down. Now you have to worry about when they will start shooting," Helmke said.

Bill Wade, president of the Coalition of National Park Service Retirees, called the new law a sad chapter in the history of the park system.

"People go to national parks to get away from things that they face in their everyday living, where they live and work. Now I think that social dynamic is really going to change," he said.

Bryan Faehner, associate director of the National Parks Conservation Association, said the law would place an unfair burden on park service employees, who will have to wade though a variety of state and local laws to determine whether visitors are breaking the law.

Officials said visitors who want to bring a gun to a national park need to understand and comply with state gun laws. More than 30 national parks span more than one state, so visitors need to know where they are in those parks and which state law applies, the park service said.

A spokesman for the National Rifle Association scoffed at the idea that parks would become more dangerous, saying people have been assaulted and even murdered in national parks.

"This common-sense measure will enhance the self-defense rights of law-abiding Americans and also ensure uniformity of firearm laws within a state," said Chris W. Cox, the NRA's chief lobbyist.

The National Park Service said there were 3,760 reported major crimes, including five homicides and 37 rapes, in 2008, the most recent year for which data was available. The agency does not note which crimes involve firearms. Crime is down across the system's parks, according to park service spokesman David Barna.

Sen. Tom Coburn (R-Okla.), who led congressional efforts to change the law, said concerns about increased violence were overblown.

"I don't expect anything major to come from this other than to restore the Second Amendment rights taken away by bureaucrats," Coburn said

The park service has prepared for months for the new law. "We will administer this law as we do all others - fairly and consistently," National Park Service Director Jon Jarvis said in a statement.

National parks hosted about 275 million visitors in 2008, the agency said.